In the Mahayana tradition, there is the notion of sudden enlightenment. I am not asking whether sudden enlightenment is possible or not. I would like to know if Mahayanists see acquisition of knowledge of the abhidhamma as part of or as a side effect of sudden enlightenment, or if sudden enlightenment is considered a lighter version of fully fledged englightenment?
I believe the abhidharma is just a systematization of the sutras. Subtism has different flavours, but I believe in Tientai your knowledge is consistent with the sutras before one hits the mark with meditation.
In the meditation schools? I'm sure there's plenty of masters who haven't got a clue about the finer ins and outs of Buddhist philosophy. If you think that makes them false, then look into Korean Son Buddhism; there seems -- to my eye anyway -- to be more of a scholastic dialogue there.
I already asked the permission from the question starter to answer this question.
According to reciting study system (Mukkhapātha), there is no actual sudden enlightenment in Theravada tradition.
The sudden enlightenment without practice is almost impossible in reciting study system. If enlightenment is easy like that, people must can enlighten without the Buddha. Actually, although the outsider Jhāna master before Buddha time can't enlighten by themselves. The outsider Jhāna mastes meditate more than 12 hours per day not a year but entire their life like Ārālatāpaso, etc.
But why there are many sudden enlightenment in Tipitaka and Commentary?
No, there aren't in reciting study system. Actually they were study and practice very hard before, but the stories were cut into other suttas or commentary to avoid of reciting too much long or to fit the Sutta for the target listener (who will enlighten at that moment).
Sutta was separated into many tiny sutta in Buddha's time because of human's memorizing ability. However, every Ariya try to append Sutta more and more through the time. Every Arahanta in KN. Thera-therī-gāthā attained 6 Abhiññā. Even Kassapa, Ānanda, or Moggallāna still go to study Sutta from Sārīpalutta in MN. MahāgosingasālaSutta.
So when people nowadays read Tipitaka, they don't know how to connect (Sandhi;Anusandhi;PubbāparaSandhi) the various tiny Sutta and commentary together. Then these weird thinking are going inside you such as "we should cut Abhidhamma off", "we can enlighten immediately without practice", etc. These all sound weird because it's not compatible with Karma when people did uncountable times Karmas before, then all is purged in a moment without any practice. It's impossible.
Sariputta was a priest before meeting Assaji and enlighten. Aññākondañña practice with Siddhartha many years before enlighten in DhammajakkappavattanaSutta.
This is why there are many story Canons in KN and Commentaries. We tried to avoid suddenly enlightenment without practice.
If you recite Abhidhamma commentary, you'll see that they mentioned 24 Buddha in the past to show how hard it was when Bodhisatta practice to enlighten by himself. The hardest part is about the reasons, relation, of everything. If Bodhisatta can't understand the reason of everything as it is, truth. He will couldn't enlighten by himself, too.
It must happen to every practitioner as well (however there are various styles depending on each one's abilities, skill experiences, past attention, etc.) If the practitioner doesn't understand the relation of elements as it is, truth, he can't enlighten because 3 characteristics appearing when seeing the truth, right relations of 5 aggregate elements.