I run a server on Hetzner that sends maybe 100k messages a week, and appears to have no problems with delivery. It doesn't even have SPF or DKIM set up. And I'm pretty sure that if messages were not getting delivered, users would complain. At least that's exactly what happened the one time there was a configuration screwup [1] that made some email providers reject mail from that server.
So I don't know if that theory explains the issue either, though it could be a component.
That's probably more true in general than it is in this particular case. Gmail does IP reputation differently than other major ISPs, and domain/content reputation are very different beasts entirely.
Gmail's approach with a low-reputation IP is generally to throttle it aggressively; mail still gets through, just not very quickly if you're sending in bulk.
Domain reputation is not initially distrusted unless it's a brand-new domain (registered in the last 30 days or so), and generally doesn't come into play unless there's very unusually poor metrics (think sky-high spam complaints) associated with the domain. Content reputation is applied in a similar manner.
The headers I'm looking at should most likely give Gmail enough distinct identifiers to work with to isolate it from broader reputation issues at Hetzner, but the post only included partial headers, so I can't quite say that conclusively.
So I don't know if that theory explains the issue either, though it could be a component.
[1] https://www.snellman.net/blog/archive/2014-12-05-how-buying-...