It took me a few times to understand their sentence correctly.

What I think OP meant to say:

All pupils found success. My success could be explained by being primed for success with 18 months of duolingo, but all pupils had similar success.

As written, it reads like "all pupils [...] primed for success by duolingo" which is perhaps why you tried to look for irony to match it with their conclusion.

If they were actually primed for success then 18 months of Duolingo should've given them a significant head start on the other students.

Judging by the tone, I'm assuming that didn't actually occur.

Studying Duolingo for 18 months should have primed them for success, but after just three weeks of an in-person class, it was evident that it had not. It's implied (or assumed) that the class was for 2nd year students or less, so Duolingo must be less effective than traditional instruction.