The distance traveled from POVUS, (where the plane had to have been at, or very close to at 2:34) to Steel's projected location is 2,535nm. From 2:24 to 8:19 is 5hrs 45 minutes, this equals 440kts average speed for that leg.
The only possible explanation for anything like a 470kt average speed on the southern leg would be if the plane flew far to the north of Banda Aceh. This can't be because of both ping ring data and fuel on board.
270nm Subang to IGARI . . . . . . . . . (405) kts average speed
510nm IGARI to MEKAR +10nm . . . (510) "
104nm MEKAR to POVUS . . . . . . . (520) "
2535nm POVUS to 36.02S, 88.57E . (440) "
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3,419nm in 459 minutes = 446.9 kts average speed for the entire flight
Note: Significant tailwinds figured in for the middle legs.
Note: If the last leg also had tailwinds, the TAS for that leg would have been even lower.
Conclusion: Mr Steel's numbers don't make sense.