I understand the point, but struggle with it tbh. Bradman's record should have been even better in reality. Missing at least four series because of the war (prime years) and also having an un-Bradman like average in one series because of Bodyline.
The only real argument that I can understand was that Bradman was not so good on 'sticky wickets' but even that argument has no merit when it comes to STR as they do not exist now.
Every era has there hurdles for Bradman it was sticky wickets, no protective gear, WW2, weaker bats and larger grounds.
For STR we have lots more cricket burnout, more variety of conditions, more planning and a greater knowledge as passed down from earlier times.
What I think is it all evens out and you have to compare cricketers against their contemporaries (spelling) which means Bradman was at least 40% better than the rest, STR not so far ahead and therefore not the best batsman ever.