That seems to be incorrect, and quite possibly originating from Tim Sweeney.

The only thing I found is that steam keys, which (as a publisher/developer) you get from steam without paying, cannot be sold for cheaper off-steam. The reason for that is obvious, since steam doesn’t get their cut on keys, but they still have to provide the support and infrastructure for those users.

If you have a source on that claim though, I’d love to see it - I tried finding anything else on it once and failed.