The issue was always large enough to impact the election by causing low turnout and abstention across the electorate.

The protest and third party voters, who turned out and did not abstain, were small enough not to decide the election.

This isn't rocket surgery like hating waffles.

Source? Your link doesn’t include this information (or even the absolute numbers of the people it polled).

Literally just click the toplines or crosstabs links at the very top of the page.

You're right about keeping an eye on sampling but you don't know much about stats if you think 600+ respondents isn't enough to draw any conclusions.

You can start making statistical conclusions from as few as 3 samples and from around 30 you're only getting square root improvements in margin of error. 600+ respondents is more than enough if there's no selection bias, which I'd trust YouGov to be able to handle.