How does acetone "rejuvenate" PEI?
Rejuvenate is probably a bit of an exaggerated term. The number one adhesion suppressor is grease. The stuff that comes off of the fingers used to handle the sheet. Even if you are careful and only handle the sides, the grease will be carried to the center of the plate next time its cleaned with less aggressive solvents.

Isopropyl alchohol does break down grease but not to the extent that acetone does. Acetone also attacks plastic particles that accumulate on the print surface over time.

PEI is resistant to a wide array of chemicals[1] including acetone but it can become brittle if exposed to it too much especially when hot so acetone is not recommended for daily cleaning. 

[1] Ultem Product Broshure table 4-3