Well, if not, the presumably nobody would make a link to the virus. If severe fatigue happened two years later it would be very hard to know if it had anything to do with a prior illness, wouldn't it? Mild fatigue after a virus would just be a bit of post viral fatigue.
There might be a bit of a misunderstanding regarding of what constitutes "severe" ME. It generally means bedbound, or at least at the worse end of housebound/couchbound.
In my case, I had a nasty infection, missed a week of class, rested another week during the holiday break, then felt pretty good and started back to class. I was walking briskly for 20-30 minutes per day at the time, I was in a language class 4 days per week and volunteering on the 5th day, and spent about two hours per week-day on public transportation. That was not even remotely severe ME.
But my legs started burning very painfully when I was hurrying to the bus after class, and that didn't stop after a few days when I expected my muscles to get accustomed to it again. My brain started cutting out in class. And when I got home I would feel sick and my skin was ice-cold for hours, and I'd be a bit crashed the next day. But I was still quite active for at least two months after the nasty infection and the onset of symptoms - at the mild end of ME.
During that time, symptoms got progressively worse. When my legs kept burning, I started walking slower and catching a later bus. That helped a bit with immediate symptoms. But OI kept getting worse, and PEM. My class ended, but I was still at my volunteer job for a couple months. I started having OI "attacks" at my volunteer job, even though it was mostly desk work done while sitting down. I'd walk to the bus stop afterward and fight to keep standing up since there was nowhere to sit. Then I'd get home and be very sick afterward for several days. That was still mild, though getting closer to moderate.
Eventually I couldn't keep doing that. But I still walked to the hair salon down the street to get my hair cut. I went to the shops with my fiance driving and walked around. I walked 500 meters to the GP's office when I had appointments in the middle of the day. I was sick afterward with unmistakable ME symptoms, but it was still closer to mild than severe.
After a year or so I was mostly housebound. I got too sick after walking to the GP, so my fiance would drive me there and back, then go in late to work. I was still sleeping upstairs, though it was a struggle getting up and down them. At some point I realized baths were making my heart rate stay elevated for hours or days afterward and switched to showers while sitting on a stool. I would garden for 15 minutes at a time by sitting on the ground and scooting around to get to the weeds. I could still vaccuum a bit, and do laundry, and go for rides to the shops or around the park in my mobility scooter. That was moderate.
Eventually I gave up on the stairs. Showers happen once every 5 days - basically when my hair gets too greasy. If I'm too sick to get up the stairs, my fiance washes my hair in the kitchen sink. Usually I'm too sick to use my mobility scooter, so we used a wheelchair for essential outings, and a couple occasions when I wasn't up to using the scooter but felt good enough to go to the shops. The wheelchair was only a 6 month loaner, so that's gone now. I sleep in the living room, and I try to tidy up downstairs during the day, but cleaning stuff isn't possible anymore. I usually can't handle any gardening. I can make dinner, but only if I start it early in the day. After a couple months of getting delivery every night, we got a crock pot (slow-cooker) so I can throw in meats, sauces, and spices early in the day and at a slow pace, then drop in a starch and pre-chopped veggies closer to dinner time. This is moderate-severe.
Severe would be bed-bound. Able to get to the bathroom, and maybe grab prepared food from very close by, but that's it. Very severe would be completely reliant on help with eating (if able to eat at all) and getting to the bathroom.
I don't think most of us go directly from having the initial infection to "severe" symptoms. We go from the infection to having weird and obvious symptoms which interfere with our lives. Then a lot of us experience a relatively quick decline during the following months. That quick decline slows down, but often continues at a slower pace. At some point, people stabilize at mild, moderate, severe, or very severe ... or somewhere in between those categories.
So I disagree with the experience being described as "severe fatigue". The symptoms weren't severe at first, and "fatigue" is not a word which ever occurred to me to use in describing my symptoms.
Language appears not to be our friend with this illness.