The rumor was started by a tabloid, IIRC and they're not known for checking their facts ;)

Boomer never explained it to the other Rogues and apparently, he feared what would happen if they learned that his son was related to Zoom. He might have been overly paranoid, but they did steal the guy's corpse just to blow it up.

As for Cold, he openly stated he didn't care that Boomer 2.0 might be his nephew because he was a rogue.

Then Judd went and screwed things up, sigh...