Wait, am I missing something here? Mantis men have absolutely nothing to do with Half Life.
Portal and 
Half-Life share the same universe. They retconned the cut ship from 
Half-Life 2 for 
Half Life 2: Episode 3 as an Aperture Science ship, which there's an Easter Egg of in 
Portal 2. Don't you remember all the references to Black Mesa in 
Portal and 
Portal 2? Cave Johnson cursing out Black Mesa? They were in competition with Aperture Science.
But I still don't think Mantis Men ever had anything what so ever to do with the Borealis. By the chronological timeline of The-Higher-The-Newer, they're at completely different times.