From where?
You must own a HA one in order to breed down a HA one. Question is, where (from whom) do you receive the first HA one?
And I'm not counting receiving a hacked one from trade. Which NPC gives you a HA Grookey? Or where do you catch a HA Grookey?
Yes, Hidden Abilities can be bred down, provided you can obtain a parent legitimately.
"The only way to get [a wild] Pokémon with Hidden Abilities this time around is through Max Raid Battles"
Don't think the starters are in Max Raid battles.. ><