Arguing does not equal lack of respect really in any context, even if it were true.
It really does. It depends on what the healthy amount of respect should be.
If I argue with you about hashkafa, it does not indicate disrespect, since a healthy amount of respect for you would not mean your hashkafa is almost definitely more correct than mine.
If I argue with my doctor, that indicates disrespect for his medical knowledge, since I am equating my ability to diagnose with his, and I know next to nothing about medicine.
So, if he thinks he can argue with the mechaber, that indicates he thinks he has an appropriate knowledge compared to the mechaber to argue. Which is disrespect, considering the mechaber’s actual knowledge.
(The same is true of Rav Moshe. But let’s stick to the mechaber.)