Overall I get the sense that the reason this is proposed to be a wiki isn't that a wiki brings real benefits to the problem being solved compared to alternatives, it's that the people involved have been working on wikis, and especially MediaWiki wikis, for a long time. It's a collective hammer and as a result everything looks like nails. Building things on top of MediaWiki isn't the only way to make a successful collaborative project, particularly one that is in the programming space.
The "hammer and nails" analogy is certainly a valid way to look at it, but for twenty years the community that writes wiki pages has been a pretty good hammer.
The main question is whether Abstract Wikipedia will be built as a continuation of existing practices with some significant updates, or as a totally new thing. Both paths are conceivable, but less continuity means more effort in building a community from scratch. Also, this eventual community will most likely be smaller, and the relationship between this community and Wikipedia will be even more difficult than the relationship between Wikidata and Wikipedia. As long as these challenges are acknowledged, it's OK to think about it.
That being said, the two aren't mutually exclusive. It can be both a MediaWiki wiki and a git repo if bridged correctly.
Indeed. It won't be trivial, but it's a sensible path to consider.