I think the criticism (just interpreting the post, don’t know anything about the technical situation) is that the complication is not necessary/worthwhile.
POSIX can be complicated, but it puts you in a nice ecosystem, so for some use-cases complex POSIX support is not over complicated. It is just… appropriately complicated.
Sure, but then you can make that argument about any of the features in Minio, in which case the parent's argument about Minio as a whole being overcomplicated is invalidated. Probably the more sensible way to look at things is "value / complexity" or "bang for buck", but even there I think POSIX loses since it's relatively little value for a relatively large amount of complexity.