If I've understood it correctly, in the MBM theory, lactic acid is not the primary cause of PEM. Lactic acid is only a secondary factor, than may further exacerbate PEM.
In MBM, their theory of the cause of PEM centers on the temporary loss of ATP molecules in the cells. They say these ATP molecules are literally lost, and that this is the primary cause of PEM (ATP molecules are like delivery trucks that transport energy to where it is needed in the cell, so when these trucks are lost, there is no energy delivered).
The body takes several days to re-manufacture some new ATP molecules, and during this time, you remain in PEM until the new ATP molecules become available.
This MBM theory of PEM is explained in detail in two posts:
this post (see the "Myhill, Booth and McLaren-Howard Theory of PEM" section), and more briefly in
this post.
It does take a while to get to grips with the MBM theory of ME/CFS and PEM. I spent weeks if not months trying to understand it. It's best done slowly I found.