Regarding the Saudi-Pakistan 'mutual defence' agreement signed last September, the talk in some diplomatic circles here in India is that either an understanding has been reached and/or assurances have been given (probably during NSA Doval's visit to Riyadh or earlier), that the Saudis will not be getting involved in any future India-Pakistan conflagration, and that the deal is only meant for Pakistan to serve the Saudis' interests, but not vice-versa. That remains to be seen in reality. The next bout of fighting in the subcontinent may not be far off so that will be tested soon.
But like you pointed you, the reverse (Pakistan treating an attack on Saudi as an attack on itself, as per terms of the Sep 2025 agreement) has already been tested and has come up short. Iran has been consistently attacking the Saudis (not just US bases but Saudis' own oil infrastructure). In light of Iran's attacks on US-allied countries in the region following last year's Op. Midnight Hammer, followed by Israel's attack on Hamas officials in Qatar, it made some strategic sense as to why the Saudi-Pak agreement came about as a means of potentially enhancing deterrence against future attacks, actual deliverables notwithstanding.
But Iran has already called this bluff. Pakistan will not be attacking Iran on Saudi's behalf either. In fact some Gulf countries (Saudi, Kuwait, UAE, maybe Qatar) have actually carried out several discreet retaliatory air strikes on Iran over the course of this war, but Pakistan has not gotten involved in those actions.
And as of the last few weeks, even Yemen's Houthis have called that bluff by attacking Saudi infrastructure, and received no Pakistani response.
I don't yet know if the new Saudi-Pak-Turkey agreement changes anything, but what is certain now is that the Saudi-Pak agreement, which was similarly worded as this one, has been a bit of a sham. If it's purpose was deterrence, then it has failed. If it's purpose was actual deliverables, it has also failed. If that agreement was aimed at creating a NATO Article 5-like deterrence structure, it has missed that goal by a long shot.
