I'm confused as well, because he wrote

> I did it as usual - updating dependencies

but later

> After upgrading the library, performance and CPU characteristics on Scala 3 became indistinguishable from Scala 2.13.

So... he didn't upgrade everything at first? Which IMO makes sense, generally you'd want to upgrade as little as possible with small steps. He just got unlucky.

It would have been a transitive dependency based on the comments about the library being "transparent" and the author unaware it was even used.

Pinning specific versions of transitive deps is fairly common in large JVM projects due to either security reasons or ABI compatibility or bugs