With regular bikes, most people start with a frame that's roughly the right size. From there, one can typically adjust the fit with different stems, handlebars, seat posts, saddles, and cranks. I think bike fit is usually about going from an OK or decent fit to something really good or exceptional.

Off the top of my head, most folding bikes have decent flexibility in the rear -- although things like setback might be limited -- but the front is pretty limited by limiting folks to moving the handlebar clamp in a line along the stem post. Sometimes you can adjust it somewhat from that line with different handlebars or aber hallo stems or an ahead stem depending on the brand/model; but there is often some tradeoff with the fold or limit to how much it can be adjusted.

So I think most folding bikes can be adjusted for a decent fit. But if you're looking for something special or exceptional -- say for riding centuries -- I think that the limitations are significant.