I think it comes down to what "we had made the switch over mid 2019" is supposed to mean. I had interpreted that as being the time the Rust implementation was p

I think it comes down to what "we had made the switch over mid 2019" is supposed to mean. I had interpreted that as being the time the Rust implementation was put into production and the Go implementation retired, with the rewrite starting some time before then, but I don't really have any strong evidence to support that interpretation (and that interpretation doesn't say anything about the actual point at which the rewrite decision was made). The situation you describe is certainly plausible, though. Shame that the chances of finding out what actually happened are quite low at this point.