How to finally disable comments on content type?
As juampy wrote, node_revision should be updated as well. You need to execute 2 queries:

Step 1:

UPDATE node SET comment = 0 WHERE type = 'your_content_type'


Step 2:

UPDATE node_revision nrev
INNER JOIN node nd ON nrev.nid = nd.nid AND nd.type = 'your_content_type'
SET nrev.comment = 0


Step 3: Clear cache