Cobb maybe would have said that about any tennis player. DeFord says that the Tennis Association did know of the rumors and feared that they'd become public. Tilden had some mannerisms that could have bothered people.
I've also seen him compared to Michael Jackson in that people knew he was "weird" or "different" but didn't know more than that. And in both cases the pedophilia may have taken time to develop. Once Tilden had passed his prime his interest in boys became more blatant, and his activities eventually led to his arrest.
I suspect the wider public didn't know and didn't want to know. They could be unforgiving if they found out (or authorities who claimed to act in the public interest could), but aside from rumormongers there was a tendency to assume nothing untoward was going on. When nobody declared themselves as anything sexually or got caught in a compromising situation, there wasn't much interest in stirring things up.
How the h... do you know? He did say it about Tilden...who he expressly went to watch...he wasn't reported to have said it about any other tennis player. Obviously, many people didn't know he was a homosexual...my point was that many did suspect, but either didn't care or didn't do anything to cause Tilden problems. Certainly many of the tennis players knew or suspected. For one thing, Tilden never showered with the rest of the players after matches. He even confided in a few heterosexual players. Would he have been run out of the tennis world if he had publicly announced? Maybe...but the fact remains many people knew or strongly suspected, and Tilden's career was not derailed until after he committed a crime.