When reading up on growing your own vegetables, I found that cucumbers, marrow, courgettes etc are referred to as memebers of the Cucurbit family.
Suddenly the name for the piece of alchemical apparatus used for distillation makes sense.
The top part in which the vapours condense is called the alembic, the bottom part that is heated is called the cucurbit and it is obviously marrow shaped and people clearly noticed that and started using the word cucurbit for it.

In fact “The book of Distillation” by Heironymous Braunschweig (First book Magnum Opus Hermetic sourceowrks no. 32) on page 14, writing about distillation apparatus says “AFter that you must have blind melmets of glass liek a gourd turned in to another glass without any pipe, named alembicum cecum.”

Or the Russell translation of Geber mentions “The nexck of which must inclose the neck of the inferior cucurbit, even up to the curved channel of the Alembick, lest what is to be distilled should flie away; and lute the alembick firmly with the cucurbit”
(Kessinger edition of the works of geber with introduction by Holmyard, page 99, the first book of the Sum of Perfection)

Now the question is, how old is the use of this term? I suspect it would go all the way back to the middle east at least, but further research will be required.