I think that is exactly what Turnbull is saying; they earned their fearsome reputation.

Anyway, on a slightly different note, there is a nice little counterpoint between the movie and the novel concerning the Bocchicchio's.
From the movie when Clemenza is schooling Mike about the Sollozzo hit:

CLEMENZA
All right, you shot 'em both...now what do you do?

MICHAEL
Sit down, finish my dinner...

CLEMENZA
Come on kid, don't fool around...


In the novel, specifically the section about the Bocchicchio clan and their history:

In any case, Felix Bocchicchio was released from prison after serving three years, went home and kissed his wife and three children and lived peacefully
for a year, and then showed that he was of the Bocchicchio clan after all. Without any attempt to conceal his guilt, he procured a weapon, a pistol, and
shot his lawyer friend to death. He then searched out the two businessmen and calmly shot them both through the head as they came out of a luncheonette.
He left the bodies lying in the street and went into the luncheonette and ordered a cup of coffee which he drank while he waited for the police to come and arrest him.