I feel you on that frustration!
Here are some thoughts:
1) I only read the abstract of the article (bad brain day), but the important thing to notice is that they didn't put IV head to head with oral fluids. They just gave people an IV and then asked if they felt better. Clearly people do feel better, but this study can't answer whether IV is better than oral, since they didn't test that.
2) The amount of sodium really, really matters. Recommendations for sodium in POTS vary from 6--10 grams daily. Some people say you need this amount in addition to your normal dietary intake. That's a lot of salt!
One liter of IV saline has 9 gm. That's like having 6 packets of ramen noodles mainlined into you. One pack of normaltye has 0.8 gm. So if you only have 1 or 2 a day, well you're right, that's not going to do anything, cause you're just not getting enough salt. I bet you're not eating 6 ramen packs a day either! Before you write off oral sodium, make sure you are getting a comparable amount. Otherwise you're testing apples to oranges. If you have a good doc, she can tell you how many gm sodium you need, which I can't.
3) I feel like a lot of these long COVID articles are reinventing the wheel in terms of POTS. It's good that it's getting airtime, but it's not always up with the history of ME/CFS research. ME/CFS researches that have studied POTS for years are not writing stuff about "What a surprise! An IV helps!" because they already know that. That's me being a little snarky so you can ignore this part if you want.
You are completely right. That is a lot of salt and I won't have been taking that much. Although I do take salt on top of the electrolyte drinks.
If you or anyone knows of any Oral rehydration solution I can get in the UK that matches the composition of those recommended in the link you posted earlier, as the ones listed there don't deliver to the UK. There's tons of electrolyte products available but the article implies they need to be in that specific ratio.