Using insider information is illegal. If you mean insider information then this is total nonsense. Sure, there are a few that do illegal things (and inevitably get caught since there is so much monitoring going on).
If you read "Lessons of a Street Addict", Jim Cramer pretty much says directly he used to call his contacts at goldman to get info on trades. Reading that convinced me that day trading is for suckers without inside information.
It's only illegal if you are an insider or got it from an insider who realized a gain on it. There's a lot of misconceptions here about what insider trading actually is.
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.