I think you are reading that wrong, go.sum isn't a list of dependencies it's a list of checksums for modules that were, at some point, used by this module. All those different versions of the same module listed there, they aren't all dependencies, at most one of them is.

Assuming 'go mod tidy' is periodically run go.mod should contain all dependencies (which in this case seems to be shy of 300, still a lot).