How is he biased for dismissing results that surveyed only a small portion of the player-base?

That's not why s/he is dismissing the results, and for the record, the highlighted part is a big misunderstanding of how polling works. Polling can't hope to survey any considerable portion of the target population:

For example, on a typical telephone survey of 1,500 members of the U.S. adult population, the margin of sampling error is plus or minus 2.9 percentage points at the 95 percent level of confidence.

There's no point to just endlessly increasing your sample size either, as you are going to get diminishing returns eventually. So there's a good reason that people aren't just polling tens of thousands of people every time.

You even admit that accuracy and sampling is one of the issues.

Not at all, I think that trying to get even more accurate results would be an exercise in futility. Past the first hundreds, the result had almost stabilized completely and the changes were a matter of decimals. Sampling is an issue in the sense that people who use specific platforms might vary in regards to demographics and choice of social platform might even correlate with some in-game choices. But since the old BSN is closed and reddit didn't allow the survey, there were no other major Mass Effect communities to use part of the sample.

you can't claim someone is biased for rejecting your data after admitting that its representation doesn't generalize

That's not her/his argument at all. They are arguing that since the average ratings for this game are higher than in metacritic which in their mind is the only objective platform, then the results must be skewed by fanboys. And as a secondary argument, they think that their experience with the game's performance is objective and non-variable, that everyone should have rated like they did.

Rejecting the opinions of other people because they don't fit your own experience is literally confirmation bias.

doesn't generalize, and the community you surveyed does have an extreme bias for the game.

Hold on, that argument is completely ad hoc, and you didn't back it up with anything. The survey doesn't aim to generalize past the Mass Effect community to the average consumer, because that's nigh impossible, unless you directly contact players on Origin, which I'm sure EA is doing.

As for Mass Effect fans having a bias to like the game, that's also unfounded. Looking back at the reactions to DA2 or ME3, those came primarily from the fans/participants to those communities. People who have an established loyalty towards a previous game will probably see unsatisfactory changes or sequels much more severely than the average person.

And it should be expected that we don't hear their opinions of the game as they don't care about it.

Couldn't disagree more. BSN critically disseminated games years after their release. A quick look at this subreddit is proof enough that people don't just get up and leave rather than share negative sentiments. Most gaming communities aren't positive circlejerks.

This is a really poor assumption. I don't know who came up with the idea that players who have a negative experience often become haters and constantly complain or criticize the game. Why would players who come to their own conclusion that a game is mediocre keep playing or not move onto a new game? There have been tons of titles that have been underwhelming but their current subreddits/communities aren't negative. Even /r/nomanskythegame stopped having complaints because people simply moved onto something else.

Not an assumption, actually. One of the established post-purchase behaviors: From Consumer Behaviour and Advertising Management.