Pete and Steffi had the same forehand technique
 

Am I right about this one, it seems to me that they had the same swing, maybe grip...

More or less. Even Lendl's was similar.

Their exaggerated "elbow-up first" take-back -- and then half-loop swing was used to be a clay court technique to do topspin drive with a smaller stroke that can last for hours yet generate enough racket head acceleration each time - with those heavier and sub-85 sqi rackets

Worked pretty well for them even on grass.

But I guess with the rackets nowadays, people can take FULL-LOOP swing with much more comfort. (Though the "elbow-up" take-back is still used today)

Their backhands aren't too dissimilar, either.

AS IS THEIR INCREDIBLE RECORDS IN GRAND SLAMS

They indeed has a similar swing, and yes, a similar grip too. The elbow-up take back is used by Eastern/continental grip users to ensure that the racquet stays closed enough to provide topspin.

Great explanations, PaulC and Fugazi, thanks!