Just like the Marines but much nastier

I ran across a JL sort of passage on page 6 of The Fall of the Roman Empire by Peter Heather:

“Recruits trained together, fought together and played together in groups of eight: a contubernium (literally, a group sharing a tent).  And they were taken young: all armies prefer young men with plenty of testosterone.  Legionaries were also denied regular sexual contact: wives and children might make them think twice about the risks of battle.  Basic training was grueling.  You had to learn to march 36 kilometres in five hours, weighed down with 25 kilos or more of armour and equipment.  All the time you were being told how special you were, how special your friends were, what an elite force you belonged to.  Just like the Marines, but much nastier.”

That last sentence puzzles me.  To begin with if the Roman legionaries were “just like the Marines” then they couldn’t be “much nastier.”  Secondly, why is this British historian writing about the American Marines?  Or is he?  Do the British Marines have a reputation like the Americans?  I hadn’t heard that.  Lastly, upon what does he base a conclusion that the Roman legionnaires were nastier than the American Marines?