To perhaps state more succinctly...
matchExactly, as stated, only advises the client to hide (ie not present) any forms/versions not in the list. It does so by adding a new flag to the existing formList API. It does not perform any synchronization of what forms are actually loaded onto the device.
From an API purest standpoint, IMO the current OpenRosa formList API is already well specified and unambiguous in terms of its intended purpose: listing the forms you can download and for which submissions will be accepted. In particular, advising the client what to do with other forms not in this list is out-of-scope of this specific API endpoint. Instead, whether to show or hide '3rd-party' forms is a client-side configuration option - of which there are potentially many - and controlling such things remotely probably requires introducing a new API.
The counter-argument is this matchExactly provides a quick (and dirty?) low impact mechanism to tell ODK Collect to hide these other forms from the user (overriding Collect's current default behavior), which doesn't require developing any new APIs.