The interesting thing about that line in Half-Blood Prince is that we know Snape disagreed with Harry about how to deal with Dementors, but Rowling never tells us exactly what Snape’s alternative was. Harry had learned the Patronus Charm from Lupin in Prisoner of Azkaban, and it was the clearest proven defence against Dementors: it could drive them away and protect people from their soul-draining influence. But the wording in Half-Blood Prince doesn’t establish that Snape rejected the Patronus Charm itself; only that Harry disagreed with his approach in an essay. Snape was an exceptionally skilled wizard who specialised in Occlumency, invented spells and developed a reputation for dark-magic knowledge, so it’s plausible he favoured a more technical or aggressive response rather than the emotionally demanding Patronus. Still, we have to separate what the books confirm from what we can reasonably speculate. My best guess is that Snape advocated a practical, less emotionally dependent approach to surviving a Dementor encounter, possibly escaping the area or using other magic to create an opportunity to get away. Occlumency is an interesting suggestion because Snape excels at controlling his mind, but it protects against mental intrusion; the books never establish that it blocks a Dementor’s ability to drain happiness or administer the Kiss. Likewise, fire spells, Shield Charms and Animagus transformations have no confirmed status as reliable Dementor deterrents. Chocolate helps someone recover after a Dementor encounter, but it doesn’t repel the creature. The official Wizarding World material explicitly describes the Patronus as the only spell effective against Dementors. So I wouldn’t confidently claim Snape had invented some secret counter-spell. The most credible theory is that Harry and Snape disagreed over the best strategy for confronting Dementors, rather than necessarily over which spell could drive them away. And given Snape’s personality, I can imagine him arguing that Harry’s preferred solution was impractical for most wizards, since producing a Patronus is advanced magic that many adults cannot perform. That last part is speculation, but it fits what we know about the spell and Snape’s demanding approach to Defence Against the Dark Arts.