I don't quite understand bumping the engine with the ignition off, unless you mean he got the engine turning first and then swtched ingintion on to enable the engine to run.

Possibly the ECU didn't like the fact the engine is running without the ignition switch going to the start position first.

A lot of modern cars whether stop start button or normal key there is a noticeable delay between engaging the starter switch and the engine turning over, presumably this is the ECU giving permission for start, or beep the horn, or turn the lights on, or anything else for that matter, doing it the bump start route probably confuses the hell out of it.

Hope that the catalyser is still OK after this strange experiment!

That is a lazy urban myth that no one would bother repeating if they thought about it for more than a nano second.

The engine cares not a jot whether it i turned via the starter motor or via the driveshafts*. In fact the ECU will inject more fuel on a cold starting sequence than on a bump start. Bump starting will obtain far higher engine speeds than cranking on ther starter motor. This means that it will start without the usual enrichment. I've bumped the engine literally hundreds of times in traffic jams to save fuel compared to the conventional cold start.

Secondly, the catalytic converter doesn't work until it gets to ~300C, at least 30 secods after a cold start by which time any unburned fuel had long since vacated the systen.

Thirdly, the ignition was in the 'I' position, so the injectors wouldn't have been spraying fuel into the engine.

BTW, the purpose of the experiment was to demonstrate pumping losses-which required the throttle to be opened on a engine being motored.

*Care is required with hydraulic chain tensioners , but that is irrelevant in this case.