simple answer. a) cabin was stocked with less booze than usual b) purser wanted a faster than usual Y cabin service and enacted this rule on the fly (my money's on this option.)

Certainly DL would sell as many drinks to Y pax that would be considered reasonable.

OP I would send a friendly note to DL.dumb. No complaint just a "here's what happened" kind of thing. curious to hear the response. For sure there was no hard/fast rule enacted that curtails the amount of booze DL can/will sell.