I’ve been wondering about what happens and it seems to me that you could have been misinformed. Google ‘aircraft cargo hold pressure’ and you should get a series of results up that indicate that cargo holds are pressurised, some are heated too. YMMV.
https://www.telegraph.co.uk/travel/travel-truths/aircraft-stowaways-airport-security-breach/ :
“Many are surprised to hear that the cargo hold in the belly of an airliner is pressurised,” Doug Morris, an Air Canada pilot, said.
“Conditioned air is directed from the cabin, so the air tends to be a little cooler by the time it reaches the cargo areas, which are also less insulated than the cabin. Cargo temperatures vary in our fleet. The Boeing 767 maintains its baggage hold above 7˚C, but the bulk area (where animals are carried) can be heated above 18˚C.
“Controlled temperature cargo bins are also available when temperature-sensitive goods are being shipped.”
Congratulations to the OP on getting their new (to them) Uke home. I hope to hear how the work on it goes.