The two features use completely different underlying technical approaches. BTRFS happens to perfectly support 'folder quotas', while Ext4 happens to perfectly support 'user quotas', which is why you see this mutually exclusive situation.
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Thank you for the answer.
From my perspective, I find weird to consider that a user is not a folder but I don't know the differences between btrfs and ext4 so I'll take your answer as it is.