Because, they confess Jesus is Christ and came in the flesh they believe in the Cross the One Triune God and also they do not deny the Father and the Son..
1 John 2:22-23
So by definition they don't fit the Biblical description of what the Antichrist will be. Which will be Anti- Cross Anti- Father and Son and I can understand why in the past others may have seen it that way but it will be anybody wether Catholic or Protestant or any other religion that denys these things. It's about denying the Historical Christ.
So based on this definition it seems that Walid Shoebat may be right. And the Roman empire was a big empire and lots of those countries now are Muslim. And now other's are saying Europe may be Muslim by 2020 if not sooner so Roman Empire can still fit into prophecy.

The Roman Empire In North Africa - Part I History ChannelDZ

http://www.youtube.com/watch?v=wycTxNjl4fo
Even Jack Van Impe has changed his mind on this with new facts and so have other ministries as well.