How does the path according to Sheng-yen compare with scholastic Buddhism?
when, according to Sheng-yen then, does the practitioner realize
non-attachment within the formless realms?

Sheng-yen taught Chan which is a synonym for Zen meaning something like meditative mind. This is comparable to meditative absorption in Theravada.
Juxtaposing how the two traditions fragment their understanding of those states is a little tricky, because Theravada shreds the meditative absorptions into miniscule details even going as far as detailing step-by-step the precise instructions needed to attain meditative absorption. Not my forte, but the Theravadins seems to like it!
Chan on the other hand takes on a more natural and organic approach. Flexibility, suppleness and malleability come to mind. In my view, this approach is much more in-keeping with suchness. There is little to no emphasis on moving through the jhanas and the arupa ayatanas. The exertion to reach those states is generally seen as a hindrance. The aim - if we can call it an aim - is that Zen realises meditative absorption in every waking moment; it is not seen as a practice confined to a zafu cushion which is more favoured in Theravada.
I know this doesn't answer your question, but instead points out how it is difficult to level the two schools such that they meet harmoniously.