Cain supported Romney only late and tepidly. He supported him because he thought he would be better and more electable than John McCain.
That’s not a stunning endorsement.
I’m 100% with you that his 2008 endorsement is not something to hold against him. However, you are falling into a somewhat common trap of ignoring what Cain actually SAYS and substituting your own interpretation of why he was saying it.
You can’t read Cain’s 2008 endorsement and not believe he actually meant it (well, I guess you could believe he was lying in it, but I don’t). Cain also continued to say similar things about ROmney this year, until Cain started shooting up in the polls. Now he mostly ignores Romney.