This is a good point I think. If this system were adopted, all it would take is a small number of activist people to understand how to game the system, then post a bunch of YouTube videos explaining to all their acolytes how they can join in and game the system.
The lived experience of IRV is that this does not happen.
There are probably a few reasons for this. Firstly, the situations where tactical voting can be of benefit tend to arise only rarely. Secondly, when these situations do arise, it is not usually apparent in advance of the election. Thirdly, even when such a situation arises and it is reasonably forseeable in advance of the election, it is easy for the tactical voting to backfire and hinder the candidates chances rather than helping them, if the vote shares/exclusion order aren't as predicted.
It's been used for single-member constituencies at the Federal level in Australia for 100 years, and in the various States for at least 50 years. French Presidential elections have been using run-off elections for even longer, and those are in theory open to the same kinds of tactical voting in the early rounds as IRV.
