According to the wiki:

"Pillagers may spawn on any valid opaque block as long as the sky light level is 11 or lower, and the block light level is 8 or lower, but can also spawn on grass or sand blocks regardless of sky light level." (https://minecraft.fandom.com/wiki/Pillager_Outpost#:~:text=Pillagers%20may%20spawn%20on%20any%20valid%20opaque%20block%20as%20long%20as%20the%20sky%20light%20level%20is%2011%20or%20lower%2C%20and%20the%20block%20light%20level%20is%208%20or%20lower%2C%20but%20can%20also%20spawn%20on%20grass%20or%20sand%20blocks%20regardless%20of%20sky%20light%20level.)

However in my testing they can spawn on top of cobblestone as well, at sky light level 15, block light 0.

Repro steps:

In the world linked above, open in vanilla 1.18.2. Use "open to lan" to enable cheats, fly over to the outpost at -350, -250.

Create a cobblestone pad below it:
`/fill -323 60 -227 -385 60 -299 minecraft:cobblestone`

Remove the outpost itself (I used a bunch of fill commands with air)

/gamerule doDaylightCycle false

/time set noon

Expected behaviour

Light level is sky 15, block 0
Nothing spawns

Actual behaviour

Light level is sky 15, block 0

A LOT of pillagers spawn