Polls always come with a margin of error, so the central value really is overstated often. In the run-up to the referendum, generally speaking no poll had the margin between the two sides very high. Also, despite the apparent impression to the contrary, rather a few polls showed the Leave campaign ahead -- and, certainly, it seemed that the general trend was for the Leave vote to be increasing as the referendum date approached.
If you account for margin of error, then, the 2016 polls weren't wildly inaccurate. They predicted a close race, probably with Remain ahead, but with a great deal of uncertainty -- which is consistent with the final result.
As for this poll -- well, who knows? I don't think it's unreasonable to see fluctuations like this, especially in the wake of the recent deal in which it looks like we've given rather more away than some people were hoping for. But on its own it doesn't mean anything.
But referendums are a snapshot of public opinion at the time of the vote, and that's all, and there is certainly every possibility that opinion either has changed, or will change, since the referendum proper.