Yep that is the point of WIDL_TIME_OVERRIDE, but I think the thrust of MIDL's change (and this one) is that there's almost no benefit to embedding the build time inside TLB files, so let's just make it always reproducible rather than requiring an environment variable.
(With a possible contributing factor being that the build time uses a 32-bit Unix timestamp, so it will be wrong starting in 2038 anyway)
And I guess the ecosystem of things that parse TLB files is fragile enough that they couldn't remove the timestamp, instead it's just set to a garbage value.