They weren't amateurs but it's fair to say that the best bowlers of his time were playing for his side.
Nah, calling bs on this.
Best spinners, certainly -- Grimmett & O'Reilly (and to a lesser extent Ironmonger) were absolute guns, but Hedley Verity really wasn't too far behind them (especially considering his stats got nerfed by, y'know, bowling to Bradman whereas Grimmett/O'Reilly never had to do that at Test level).
But FMD Australia played some absolutely rubbish pace bowlers during Bradman's time. Pre-war, Tim Wall was the only Australian quick to hit 50 Test wickets during Bradman's career. The next highest wicket taker during that period appears to have been Stan McCabe (tied with Ernie McCormick). Now Australia wasn't the easiest place to bowl pace in that era, but in that very same era you had the following English fast bowlers running around:
Voce, Allen, Tate, Bedser, Bowes, Farnes, and some bloke named Larwood who was, in my opinion at least, the GOAT until Lillee and Marshall came along to challenge him. Not a bad pace bowling unit.
And they were ably supported by a spin attack of Verity, Robbins, Peebles, Jack White and latter-day Tich Freeman, as well as very early Jim Laker (who didn't do too well) and I suspect in any other era Doug Wright would've had stats that looked significantly better than the numbers that ended up next to his name. Only Verity was good enough to challenge the levels of O'Reilly/Grimmett, but that support is pretty solid.
Meanwhile Australia's depth bowlers on the 1930 Ashes tour were Alan Fairfax (who opened the bowling and batted top 7), Percy Hornibrook (who, by the sounds of things, was more a spinner than a quick) and Ted a'Beckett. Bit of a drop-off from the 1921 tour where Jack Gregory and Ted McDonald were ripping up.
Then the West Indies had Manny Martindale and Learie Constantine along with Herman Griffith. Even India had two passable seamers FFS, in Mohammad Nissar and Amar Singh! And then NZ put out Jack Cowie.
On pure stats taken out of context, Australia was literally the worst fast bowling unit on the face of the planet between about 1930-38. Certain English counties put out stronger pace attacks!
So yeah, two bowlers who played for Australia in that period were undoubtedly the best in the world. But the rest sucked in comparison to the attacks England was putting out. Bradman being so good devalued his own achievement, because he completely wrecked the reputations (statistical and otherwise) of the very, very, very good bowlers he came up against.