Assistance devices for disabled people are their own thing in the code (wheelchairs), and are (as they always have been) allowed most places. They are generally defined in such a way that they are much more restrictive than ebikes. The definition specifically says they must be "a device designed solely for use by a mobilityimpaired person for locomotion that is suitable for use in an indoor pedestrian area". Ebikes are not solely designed for mobility impaired people.
Also note that the ADA does not apply at all to trails designed for hiking/mountainbiking/etc. Which is logical because basically no natural surface trail is suitable for wheelchair accessibility, and really the only way to make one so is to pave it.
If "I need help to get around this trail so you are required to let me ride what I want" were a thing, people would have sued for access via dirtbike/atv/whatever decades ago.