Hol' up. If Texans can distinguish the way other Texans say "pen" and "pin", then surely they aren't actually merged? Yes, the vowel sounds are very similar, and perhaps Yankees can't tell them apart, but they must be different.
That's just the way with mergers, though: we don't attend to differences that are unimportant in our personal grammars except through conscious striving. I'm the same way with marry/merry/Mary: I know there are people who say these differently, but even if I'm talking to someone who does, those words are all the same to me. The other speaker may be giving me one of three distinct tokens by their interpretation, but whichever one it is I interpret it as one of a set of identical triplets and rely on context to tell which meaning was meant.
Or to give another example: many speakers where I grew up (Pittsburgh) reduce the diphthongs in "tire" and "tower" so that both of these words sound like "tar." In other words, three words share the same token. But I've never misinterpreted a strong-accented Yinzer's "tar" as "tire," "tower" as "tar," etc. That doesn't mean a Texan would be wrong to say that some of us pronounce these words all the same: we do.