As far as "cassette" being used in other websites: In those that I checked, the products were all described as being made in France, and at least one gave the name of the French manufacturer. Sadly, I never learned French, but I know enough to understand that "cassette" basically just means "little box" (it also helps that the English dictionaries list the French origins of the English word).
Here's the site for the French company that offers "implant cassettes":
It appears, then, that "cassette" is a false friend between French and English in this usage as well.
The producers that use that on their non-translated websites are simply parroting what the manufacturer's term for it is.
My experience with "set" comes from working with translations for other companies providing tools for implants that do not "Kassette" in German, but who describe their products as coming in sets.
But even more authoritative is Marianne (BE), who is basically the expert in all things medical here. (I was relieved when she confirmed my description of the usage.)
Cassette/Kassette remains a false friend in this case (and likely most cases).