Waqaftu and ʿaraftu are first-person past verbs: the speaker first stops and only then recognizes. Between them, min baʿdi ʿishrīna ḥijjatan makes the long interval explicit.
The accusative expression fa-laʾyan stresses how difficult the recognition is. Omitting its force would turn the line into an instant recollection, whereas the poem depicts stopping, examining, doubting, and finally identifying the site. The sequence gives the scene psychological depth.