Xe misconstrues how the standards process works, a bit. Standards don't develop out of thin air. Usually they standardize some sort of existing practice, and in this case that existing practice was DEC VTs.

In addition, there was ECMA-35 as well as ECMA-48. ECMA-35 provides some important context to ECMA-48, and the reason that ECMA-48 appears to not concretely define some stuff is that actually that stuff is in ECMA-35.

vttest post-dated these standards by three quarters of a decade for the latter and by a dozen years for the former, and wasn't really a comprehensive test suite until the 1990s.