JIS and ISO square taper I've read Sheldon Brown's page on JIS and ISO and have read exchanges in forums about the pros and cons of using ISO cranks on JIS bottom brackets.

If my understanding is correct, these standards define the shape and dimensions of the BB spindle and thus have implications for the cut  made in the crank that must fit on the spindle.

The reason for my question is that a vendor of ISO cranksets  is telling me that their cranksets will work on an ISO or JIS bottom bracket if the bike is a folding bike.  Does whether the bike folds have anything at all to do with this compatibility between BB and crank?
Does whether the bike folds have anything at all to do with this compatibility between BB and crank?


Executive summary:

NO!

Longer answer:

Here's the Sheldon Brown link:  https://www.sheldonbrown.com/bbtaper.html

Even though both ISO and JIS have the same taper angle, ISO spindles are narrower than JIS spindles, so an ISO crank won't go all the way onto a JIS spindle.

Depending on the specifics of the crankset and spindle, how far the cranks go onto the spindle may or may not be sufficient to both be safe and able to withstand usage for a good amount of time.

But I wouldn't do it for any bike I might ride - I view the situation you've described as a somewhat unethical attempt to make a sale.  It might work, but saying it will just because the bike folds is, well, sleazy.

Because whether or not the bike folds doesn't matter.  The specifics of the bottom bracket spindle and the crank itself matter.  And a crank where the design prevents full engagement of the load-bearing surface with a close-but-not-quite-right bottom bracket spindle isn't a configuration I'd be able to recommend with a clean conscience.