The MOT inspection manual is available online should anyone wish to look.
(b) a suspension joint dust cover:
(ii) missing or no longer prevents the ingress of dirt etc, - Major defect.
I do agree that although every MOT tester should follow the inspection manual, everyone's assessment ability is different - there will always be differences of opinion as we're not robots.
If you ever have doubt if an MOT has been carried out correctly, you can contact DVSA to investigate.
Personally I'd be most concerned about the corrosion and also the excessive oil leak.