Isn't a different interpretation that a fork is, presumably, their software _plus_ your modification(s)? Thus, a fork is not "solely" their software anymore and is allowed.
The issue people are (rightly, imo) pointing out is that forking without modifications—say, to remain locked to a particular commit for whatever reasons one may have—appears to directly violate the terms of the custom license (which itself appears to directly violate the terms of Github's TOS.
