What is a 'pernicious' wrong belief?
First of all, the commentary to that sutta doesn't use the term 'niyatamicchādiṭṭhi' for the sixth abhithana, it uses the term 'aññasatthāruddesa', which means 'proclaiming another teacher', i.e. setting oneself up as a student of a teacher with views outside of the Buddha's teaching, something an enlightened being cannot do (abhabba kātuṃ).
As to what the term 'niyatamicchādiṭṭhi' (pernicious wrong belief) means, the MN tika says:

ahetukadiṭṭhiakiriyadiṭṭhinatthikadiṭṭhiyova niyatabhāvaṃ pattā niyatamicchādiṭṭhidhammā.
Views such as the view of causelessness, the view of non-action, or the view of nihilism, when fallen into a state of certainty are called states of pernicious wrong view.
(MN-t 22)

Basically, the meaning is that any wrong view becomes 'pernicious' when it is clung to as a certainty.