The sample size is just fine, actually, it's very large.

The issue is randomness. If it's a random sample, it's very large. If it's not a random sample, it's worthless.

"We have no idea how they found the participants or controlled for bias in the selection process."

A) This has nothing to do with sample size

B) If they have any credibility whatsoever, they hopefully know what they are doing. Adjusting is a statistically legit thing to do, they do it in political polling (and in hindsight) it works.

But yes - with 'online surveys', it's hard.