Board Thread:Warhammer 40k General Discussion/@comment-8881468-20131229020408/@comment-3580035-20140103031248

That incident was with the Iron Hands, not Salamanders, and the Lion still did not know whether that group of Death Guard was actually traitorous or not. It was a splinter warband.

It also clearly states in that short story that the Lionel is loyal to the Emperor and the Emperor only. He doesn't care about the machinations of his brothers, but serves the Emperor in all things.