Well, it will work, but not "perfectly". You will encounter some anomalies if you play without Steam. And you eventually seem to lose access to your DLCs if you don't check in with Steam, periodically.
And, iirc, Paradox elected to put up a warning that the game may not function correctly if you run in off-line mode, rather than fully support a Steam-free option.
Can you provide supporting evidence for either of those? They don't ring any bells.