If one is a monk, one has the responsibility to learn the vinaya. If one breaks the rules even without knowing there was a rule that was broken, there can be extreme consequence such as the heavy rules. But the smaller rules can also be a blockage to attainments.
Then why ordain?
Because monk life is set up to give one plenty of time for practice. It is a net gain.
Much of the abhidhammatthasaṅgaha, which is a ṭīkā, would need to be memorized along the way as one meditates and progresses. It does not take long if one is personally working with those dhammas in meditation. The abhidhammatthasaṅgaha, has many of the core terms all nicely packed into one small book. That is why much of it needs to be memorized.
I don’t know anyone who has studied the whole of the abhidhammapiṭaka who would criticize the abhidhammatthasaṅgaha. When we get praise like that, then it is definitely a worthy ṭīkā for acceptance.