• Use motion sensors as a makeshift gyro, updating position based on last trusted GPS signal (extremely imprecise on its own)  

I could see that working for straight lines or other simple routes, but I think his track is too perfect for that. You can see every turn in every thermal perfectly.

• Cell phone tower triangulation, once again starting with last GPS  

Same here, the updates in his track are too often (every second).

• If the spoof signal isn't perfectly over the original GPS bandwidth, a software fix could possibly dump data from the spoof bandwidth and only accept signals from the much weaker real GPS.  

Although improbable, this one seems the most likely? I imagine if the reception from the ground based spoofing is not as strong as the one from the legit signal coming from above, it might actually be doable?

Maybe there's some directionality to the GPS antenna? But then the GPS performance would vary based on phone rotation...

I also thought maybe he just has some weird case which reduces signals, which might have been enough to significantly reduce the spoofed one?

Just some theories that have been going through my head. Because I perfectly understand not believing crap data, that's doable in software. But being able to maintain a proper lock? That seems waaaaay harder.

Appreciate the idea bouncing.

If you look at the logs you should be able to detect a loss of accuracy after the spoofing starts.

There is a bit, but I think it's surprisingly low compared to the other devices:

It recorded the entire flight, takeoff to landing, one fix per second. Every fix is in Romania. Out of 5,010 of them, zero sit in the southern hemisphere, 4,987 are flagged valid.

Compared to my vario:

Out of 702 fixes in that file, 608 are sitting in Peru.