rank doesn't mean shit other than status.
Sounds like you either didn't pay attention to the manga or forgot most of it. There's been plenty of instances where a demon's rank have been used to gauge their strength.
1. Kalego when he revealed with pride that he and Balam were the only rank 8 teachers in the school. Kalego values strength over status, so there's no way he'd have pride over something that didn't have anything to do with strength.
2. When Kalego, Balam, and Opera oneshotted the fusion monster at the park. The terrorist theorized that "High Ranking" demons must have been the ones responsible.
3. Kirio, a demon that's a ranked 2 despite being in his third year and from a well established family has noted that his low rank is because of his lack of demon magic and weak physique, aka he's lacking in strength.
4. When Alice was fighting the fire teacher during the heart breaker exam, the teacher was actually mad and questioned why was Alice still a rank 4 when he was so strong and stated that Alice should be higher ranked. If it was about status like you think, then the teacher should've either been mad before fighting Alice or not mad at all since. But instead he got mad after he fought with Alice and saw his strength which means he thought that Alice rank should've been higher due to Alice's strength not status.
There's some more, but I'll leave it at that. Now you may think that it's shit that Iruma doesn't get a rank up, but Iruma needs to show that he has the strength to hold rank 6 or higher. And if you still have doubts then tell me how many demons have high ranks despite being weak? If you can't, then that proves that rank does in fact mean strength whether you like it or not.