Why shouldn’t they have come up with the idea of moveable type, or some kind of stamp? Is it impossible there was more than one kind of Linear B?
Linear B is a system of writing, and it was exclusively used to record the Greek language (as well as numbers, as in inventories and stuff). The Phaistos writing isn’t Linear B. As you bring up, it’s a little strange that no one else seems to have used anything like moveable type in that era, other than the Phaistos Disk. Inscriptions on the Linear A and Linear B tablets were made with handheld stylus on clay, and the tablets may have been reused (this is known to have happened with the older cuneiform tablets, as well as much later palimpsests in medieval Europe).
Another thing that’s kinda odd is, the Hattusas archive in Anatolia contains references to the Mycenaean Greeks (that’s now, finally, accepted except by some elderly diehards, and most of them are in England), but AFAIK, not a single example of either Linear A or Linear B has been found there. Similarly, AFAIK, no cuneiform tablets were found at Pylos or on Crete, and part of the Hattusas archive consists of court copies of correspondence sent to various places, including Mycenaean regimes.