Your middlebrow dismissal doesn't work here because the parent didn't say it's not a problem or that it doesn't happen. The parent is refuting a point that successful traders exclusively become that way by trading on illegally obtained information.
No it isn't. The original comment might not have said "all successful traders spend their time...insider information" but the implication is there as it was stated. Given what the grandparent comment replied with, it appears I wasn't the only one who inferred that message.
Sure, there are a few that do illegal things (and inevitably get caught since there is so much monitoring going on).