That's a clever idea; home "ear impression" kits have been available for quite some time now, but this packages it on a headphone and uses UV cure to make it much more forgiving on timing. I wouldn't recommend it simply because the earmold is one piece with the earbud; if the earbuds die you are stuck throwing the whole thing away, while an aftermarket earmold, you "socket" the earbud into it, so you can swap out the earbud with another of same casing if one dies or you want to upgrade.
I've also seen many people offering "custom molded earplugs" using similar kits at gun shows:
https://earinc.com/product/ez-impression-kit/
I generally don't recommend them. 2 reasons - one is safety, a proper earmold needs a deep ear impression, which has real risk of getting molding compound close enough to the eardrum to cause damage. You really need a foam blocker to prevent this. The other is performance - a proper ear impression needs to be open mouth (bite on a 1" block as you take the mold and don't move until it sets) and needs to be proper depth to seal, and the molding technique affects how well they fit.
I always go to an audiologist to do mine, and even with those professionals they've had to redo the earmold once or twice due to poor fitting results. If you're comparing to generic eartips, even a poor earmold will be a huge improvement, but if you're going through the expense and hassle of a custom earmold earphone anyway, then I'd advise paying the bit more to be sure of a great result.
Photo of one of my earmolds - you can see the foam blocker and extraction string, and just how much of the ear canal gets molded.