I believe the GIRM requires a sign of reverence before one accepts the Eucharist, whether it be under the form of the Host or the Cup.
It does not, to the best of my knowledge, say anything about a sign of reverence to the form not received when one receives under only one form.
I don’t know if your priest actually said that one is to bow towards the Cup even when not receiving, or whether he may have said that one is to bow to the Cup too, meaning that one bows to the Host when receiving, and then bows again to the Cup before receiving that.
The latter would make more sense, and it is possible that someone could misunderstand that one has to bow to the Cup whether or not one receives from it, when all that was being said was that one is to bow to each when one is receiving each…
If your priest is requiring anyone passing in front of the Cup to bow towards it even if not receiving, then it would seem that he is requiring something the Church does not require. Having said that, it doesn’t seem to be something to get one’s nickers tied up about. Since the Church does not appear to require it, it would seem a bit over the top to say that it is necessary. Nice, yes, reverent, yes. Necessary, no.