Or then built for MACO, and given to UFP Starfleet later on. Or then built purely as a non-operational engine testbed, and given to UFP Starfleet later on.
For all we know, she comes from a timeline where NX-01 never reached warp 4, but was lost on her first mission for unknown reasons - in reality because of the temporal war going differently in that timeline, but Starfleet thought the Warp Five Engine was at fault and began favoring the competing project that resulted in the Franklin.
Speculative, "official" and canonical backstories are all okay, until and unless the latter contradicts the first two sorts. So far, we just have very little canonical backstory to go by...
Indeed.
Timo Saloniemi
I thought in the Alternate universe of these movies, Enterprise was the only series to remain unchanged as far as continuity and history?