It would mean that despite his hatred of an illegitimate son, he kept him around for political value. Tyrion was everything that Tywin wanted in his son.
But instead of dwarfism being the reason hate him, it's the fact he ACTUALLY wasn't his, but Tyrion still picked up all Tywin's traits.
For me it humanises Tywin and makes him more of an anti-hero than outright villain. Yet again, just like Tyrion.
I'm not particularly bothered if it turns out not to be the case, but it's interesting to discuss.