Sounds to me that the airline webpage has additional (read: useless?) security features that Expedia doesn't have.
The unambiguous part is that Google Flights failed to account for the nature of the seats it had identified as "available", and went ahead with the recommendation when in fact they were not appropriate. The unclear part is whether Expedia did actually find the same flight and seats (as the author appears to be saying) but has some way around the security features (an agreement with the airline?), or whether Expedia was aware of the inappropriateness of the seats Google Flights recommended and in fact provided different ones on a different flight.
It would be nice if the parent had been clearer in the explanation but it seems that either way: Expedia 1, Google 0.