He might not expect it, but he certainly would have been prepared for it.
"Right lads, we're gonna go in here, level one of Lorgar's greatest works, embarrass and prostrate him in front of his legion, and I'm pretty sure he won't lift a hand against us. Not sure about Malcador though. He might get it." Sorry, doesn't work for me. By his very nature he'd have come up with several plans to counter any action Lorgar might take within seconds of being given his orders. He wouldn't be able to stop himself. Things like that are at the very core of his being.
And I agree to An extent. Lorgar is a loose cannon and guilliman may have been cautious, but I still don't think merits guilliman creating theoreticals on how to fight astartes. Yes this may be naive on GUILLIMANS part