So, Dr. Frank Pyke was good enough to get back to me, and according to him:
The speed was measured out of the bowler's hand.
His words, straight from the guy who did the measurement. That's that.
So to me, it seems that Thommo was as quick as anyone we have now pre-injury and possibly faster, if he was still bowling 148kph post-injury, as everyone agrees he slowed down after. However, it does put him more into the reasonable range, as well as the rest of the bowlers instead of having 10 bowlers who were all supposedly capable of bowling 150+ in the seventies. I have no idea how the 'we take average speed' myth got started though, as it seems to be a common perception.