I don't want to start any argument here, but you kind of missed the point. Your solution only validates because the validator ignores comments. So why would it complain about your use of an iframe when it is commented out?
iframe is not permitted under Strict, which is what the original poster found out and was trying to find a solution to. The object tag was the appropriate solution/tag as that is permitted under Strict, however, at the time of the original posting IE didn't support it very well (it might not still -- I'm uncertain about that).
Either way, the proposed solution you have, will pass a validator, but is dependent on the browser running conditional comments to inject an acceptable tag if not IE, and an unacceptable tag if it is IE and it definitely does not abide by the rules for Strict, it does abide by Transitional though.