Then the whole argument becomes that the “welcome back” will implicitly indicate to the user he clicked a login button and not a signup one 
He will just have to adjust by clicking the “create your account” link at the bottom right. All is well.
Will that user be traumatized by the “welcome back - log in to your account” he just saw? I honestly don’t think so, but that’s just my opinion. No offense, but this does give me some “first world problem” vibe. I find it harmless and unimportant, but I can understand if you feel this is important from your POV (is it, though?). Cheers.