Cheat sheet or adlibing - there could be a grain of truth behind the claim.
There were some reports last year of pent up demand making stock go pretty low for some houses for some qualities.
As an airline, Finnair probably needs to get an allocation slot also for champagne. And they probably want a steady stream of deliveries, not the full year deliveries at once. So multiple slots needed over the year and failing to get the needed production slots could well be interpreted as "there is not enough supply" .

If I speculate, Finnair could have cancelled production slots in order to buy less champagne during pandemic and then they scaled operations back up again they either were to late to the game or for some other reason failed to get the production slots they needed.

OTOH, Finnair has never officially claimed or hinted that they had gotten into a supply/demand issue with JP. All communication has hinted cost reduction is the main reason, so I am still leaning towards someone did make up a "plausible excuse". Just saying there could have been a production slot issue that started the whole process.



*) One interesting this is that the head of alcohol procurement at SAS has posted at businessclass.se about the thinking inside SK when it comes to champagne; like their "special club" idea that sadly had to be abandoned due to pandemic but how they still have a serious process to pick champagnes and why they serve a sparkling wine as pre-departure and a champagne with the meal. And that they plan to have high quality champagne in ,375 bottles on shorthaul (where there is no business class cabin), only you'll need to pre-order it.
The refreshing thing is that they do have a thought process that seems to be quite different from Finnair's.