I think the more appropriate response (and I do think the help center wording should be edited accordingly) would be to say that for "feature requests, voting can indicate [...]" rather than explicitly mentioning tags.
An alternate wording, proposed by Cyborgx, that is a bit broader and covers proposals for things other than feature requests, that I quite like is:
On propositional posts, voting often indicates agreement or disagreement with the proposal rather than just the quality or usefulness of the post itself.
If a post is inherently requesting a feature, people tend to vote on it based on whether or not they agree/disagree with the feature. They don't go check the tags first to make sure it's not tagged "discussion". I don't think that that's wrong.
As it is, it simply encourages people to intentionally mis-tag a feature request as some other tag in an attempt to avoid downvotes. This just makes it harder to find feature requests, can give the false impression that they are asking about how the current system works rather than that they are discussing a proposed changed, and brings up the issue that you have brought up here, of what to do when the tags change. (There is also the related question of what to do when the OP changes the question from support to feature request or vice versa; it means a whole bunch of people "should" be going back to change their votes, which simply won't happen.)
If a post is requesting a feature, people are going to vote on it based on disagreement (at least at this point in time) and the tags used aren't going to change that. The meta wording should reflect that for the sake of everyone involved.
It's also important to note that the given help center verbage isn't stating what the policy is; every user has the right to vote however they want, using whatever metrics they want. The point of that verbage is to help users understand how many members of the community choose to vote, so that they can be prepared for the fact that it is (sometimes) different on meta. So it's not a matter of changing the wording and then forcing people to follow it; they can and will vote however they want. It's a matter of adjusting the wording so that it explains how people choose to vote.