It’s not about military strength, it’s about cultural sway, something the Greeks have in abundance at this point. Greek-speaking traders and diplomats from Magna Graecia to Baktria will take their toll, not to mention the native population. This is almost the height of the Hellenistic period; unless the Celts embark on an epic destructive rampage (which is possible, I suppose) I don’t see them stopping it. The Romans certainly didn’t.
There is a counter argument for language, though: the Galatians, the OTL Celtic settlement, kept speaking a Celtic language until the 6th century. What is unclear in this scenario is whether the Celts would just settle down in a small region of Greece or attempt to administer the whole thing. If the former, I guess some region like Thessaly becomes TTL’s Galatia and speaks a Celtic language for a while. However, odds are they eventually switch to Greek. If it’s the latter situation, Celtic becomes an administrative language, the Greeks view the Celts as foreign oppressors, and a native uprising kicks them out completely.