Linear B also maps poorly to Greek as far as I know. It's clearly an adaption of a foreign script.

Linear B maps onto Greek quite well once you understand that it is a syllabary, not an alphabet. The fit is imperfect in specific, well-understood ways.

Once you correct for well-understood systematic gaps, though, the underlying grammar, vocabulary, and even some divine names (Poseidon, Zeus, Hera, Dionysus all show up) are recognizably continuous with classical Greek.