I always have an opinion.
I would suggest that the duplication speeds back then were lower and closer to 1:1 so even with the poorer tape, heads and electronics it still was a net gain.
I have also been told that modern machines will reveal more of what was on the tape than the playback electronics and heads would reveal back then, so older tapes had in effect a better recording than they thought they put there.
Last, jazz is just a bit more simple and although it might have a lot of dynamics, the number of players is fewer and the environment is not really quite the same as a symphonic recording, so it is in a sense easier to get an excellent jazz recording than one of a large symphonic work for any given recording process.