Correct me if I'm wrong but isn't the VHF ACARS transmission uplinked from a ground station to the Inmarsat anyway so the fact that it is eventually received by the satellite doesn't necessarily mean it was sent by SATCOM?
I'm struggling with the concept too, but imho Chris McLaughlins statement only makes sense if there was some kind of SATCOM between the inmarsat satellite and the
A/C, at
1:07 which was used to calibrate the signal return path between them.
An ACARS message between ground station and satellite wouldn't doe any good to calibrate the path between satellite and A/C transceiver.