Microtransit and Flexible Transit are similar, but not similar. Microtransit is the vehicle(s) or property for serving a small group of specific people–the size of the geographic area is not defining. Flexible Transit is a strategy whereby a well-planned transit process serves a small region. Both can be, and should be, on-demand services. But Flexible Transit should never have a goal of increasing ridership since an increase in demand would trigger the need for fixed route service.

Jarrett Walker talks about this in his What is ‘Microtransit’ for? article @ humantransit.org:

So attracting many riders to flexible services is the last thing a transit agency should want to do.  In fact, when flexible services become too popular, they have to be turned back into fixed routes.  Imagine that a flexible service covering [an] area  got so popular that you needed three vans to run it.  At that point you might as well just run a separate fixed route for each [area], at which point each one could be reasonably straight.  Still, though, three buses may be more than this particular area deserves, when you look at the total budget for coverage services and spread it over the whole region.  So if you really want to claim that you’ve covered all of these [areas], you want flexible service, but you also want to take every possible step to keep ridership down.

As an FYI, Hampton Roads Transit (where I am a Governing Board Commissioner) is undergoing a multi-year study to transform the regional transit system of Hampton Roads, VA (Norfolk, Virginia Beach, Chesapeake, Portsmouth, Newport News, Hampton, and possibly incorporating the needs of Williamsburg and Suffolk). The resulting proposed plan will contain some variance of the Flexible Transit model. Information on the Transform Transit project can be found at transformtransit.com.