Well, you weren't in the classroom where he set the alarm to go off, were you? So it's not like you're in a position to reasonably judge the reaction.
Suppose he did just take apart an old clock and piece it back together for fun, and didn't specifically try to make it look like a bomb. What was the purpose?
If it had been me, I would have been eager to explain why I chose that particular case. Like maybe-- look "how portable this is" or "look how you can hang this on the wall now" or something like that. Or maybe even just-- "well I was messing around and just happened to have the case so that's why I chose it." I also would have been up front about the fact that all I'd done was dismantled and re-assembled an old clock. I wouldn't have tried to pass it off as an original circuit-board project of my own.
There's nothing like that in this interview: https://www.youtube.com/watch?v=3mW4w0Y1OXE
Also, as the OP notes, Ahmed makes a comment about using a cable to fasten it shut to make it look less suspicious. OP wonders why would he have been thinking something like that, if he did not know that a suitcase would in fact be suspicious? Sure, such a comment might have been made as some sort of back-rationalization after the whole thing blew up, but along with the other red flags it's a suspicious statement.
And if he did deliberately try to make it look like a bomb and did nothing to assure his teacher and fellow classmates that it was not real, then it's perfectly fair to call it a hoax.