Yes you can't really do one without doing the other but that point presupposes that you care who the other party is. Lots of people honestly don't. If the Lib Dems were in the position that Labour are in I don't think there would be that much difference in voting intention, except that all the people who intend to vote for Labour would be voting for the Lib Dems except for any constituencies where Labour were more likely to beat a Tory.

People are not voting for Labour because of their revolutionary ideas (unless you count basic competence as revolutionary, and even then it's only theoretical). People who are voting tactically are doing so against the incumbent, not for any particular opposition. They will vote for whichever party has the highest chance of dethroning a sitting Conservative.

Ultimately there is no reason to recommend Labour over any other party unless they were Labor party members. Tactical voting is always against the incumbent, not for anyone. You can't tactically vote for a party, except to just vote for them. But you can tactically vote against a party. So the original accusation that both OP made and in the linked subreddit is both nonsensical and utterly without evidence.