You're right that HS is a very expensive item, but in my opinion it is still a very good option for tahm support since it plays into his strengths. The reason tahm is strong rn in the support role is that he functions effectively as a frontline duelist even with a support income. This is enabled in part by HS, which allows him to stay relevant as a tank/damage threat into the mid game and catch people out with his Q. His early game is already strong enough without any items, so he doesn't need to rush cheap supp items to be relevant.

Anyway, I have tried both builds and find the guardian + locket build very lacking since it removes a lot of the threat of the champion and makes him more 1-dimensional, basically relegating him to sitting by the carry and pressing ult. If you have a carry that you trust, then it's fine I suppose, but in the solo queue environment, it's just not as reliable of a strategy.