Threaded Diablo flap disc doesn't leave room for arbor nut

Did I buy the wrong size thickness given my arbor length? First time needing this type of disc (only have used cutting wheels so far) so I'm wondering what I'm doing wrong. I'm assuming the nut has to be installed so I'm posting here first before running this tool unsafely.

TYIA

PS: I checked product page and it looks like this disc is meant to be quickly tightened using its own threads and the flange is unnecessary. To my surprise.