Another point is that folders - if genuine - often have internal batteries that make it unobvious that they are electric
I assume you mean built-as-e-bikes, as conversion kits like Swytch's are also genuine and very much obvious if you know anything much.
I don't think that's generally true, either. Many brands (example: Byocycle) have obvious batteries behind the seat post, or on a rear rack shelf, same as conversion kits, plus they have ⚡ symbols on the bike. Even the Brompton C Line has the word "Electric" emblazoned on the battery bag, in bright blue at least sometimes. They're much easier to identify as e-bikes than some of the
Ribble ones with batteries in the bottle cage. I guess people will still be taking such bikes on TfL, but if it catches fire, they could be charged with breaching the conditions of carriage in addition to whatever else.