By information, they are just better really and by showings.
One quote doesn't make someone better than someone else. It's nice that Sith Lords will lay down their sabers and surrender to a Guard and it's nice that the DC fear the whole Guard...
But that isn't the end all be all tbh, gotta look at more than just 1 mere quote. If we went by just 1 quote on everything, then we could just pick whatever quote we wanted for anyone and not really get anywhere.
Also Jango killed about the same number of Jedi with just a rock, some sand, his whipcord and his bare hands.
Well the swtor codex says that they're "The ultimate non-Force sensitive fighters in the Empire", which included the Mandalorians. And that they're "unmatched in martial skill".
Fett is also one of the best Mando's ever. Lassicar was a standard guardsman. Also those were Jedi of unknown rank and skill, whereas Sith Lords are all high level.
Said Jedi though before, were wrecking the group of Mandos that Jango was with, so they aren't featless unlike what is said about the Guardsmen killing Jedi and Sith Lords.
Yeah the Guard are the ultimate Non-Force Sensitives in the Empire, that's great and everything but...that's kinda a given considering where they stand in the Empire. Plus the Mandos aren't really apart of the Empire, they are just allies. So saying the Guard are the best in Non-Force Sensitives in the Empire doesn't mean they are better then the Mandos, when said Mandos aren't apart of the Empire government.
Also yes they are unmatched in martial skill, but it's questionable weither the unmatched skill means them as apart of the Empire or as a whole.
If it's as a whole then fine, but this just goes back to the point of the Mando having the range advantage to aid him.
Also a title doesn't make one all powerful, having the title Sith Lord doesn't mean squat if they don't have anything to show for it, the same with having the rank of a Jedi Master.
I just feel the Magnaguard being superior via showings and what they have. Plus their durability is better than a Guardsmen.