Argentina still has to qualify despite being the defending champions?
Unfortunately, yes. Argentina will have to qualify again and go through with the traditional process just like every other team despite being the defending champions and the only exception are the hosting countries namely, United States of America, Canada, and Mexico.
In the past two decades or more, every defending champions have the right to be qualified automatically but that rule changed way back 2006 before the FIFA 2026 commenced.
Now you mention it, someone could argue that it can be a lil bit unfair to have three teams which have been guaranteed to qualify only because they are hosting the event in their countries. Having one team to have a status is completely different than having three of them at the same time, in my opinion.
Still knowing how good Argentina is, I would not expect them to lose the chance of partaking in the next world cup.