Plus, the production lines for Atlas V have already been retooled with those for Vulcan, so even if they did have the engines, they couldn't make the rockets.

IIRC ULA did have a license to manufacture their own domestic RD-180s (or was considering getting it), but the factory for those would've been very expensive and not worth it with Vulcan in its early design stages.

I don't think the line has been retooled. They still have to build a number of Atlas 5s for Amazon. They don't have all of those in storage.

That's a very good point, I could've sworn Tory Bruno had said that the line had been retooled and all the remaining cores were in storage, but the closest tweet to that seems to be referring to having the engines in storage.

I think he did say the line was retooled, but the factory is used for different rockets at the same time. The Vulcan and the Atlas were always in production at the same time.