That’s quite a distance and yet it painted the location of the plane at the right distance at 1:11 and 2:11 as verified by Malaysian radar.
I think Inmarsat sent another plane on the same course, then they gathered the ping data from that plane and compared it to MH370 to double check their analysis.
They actually said they analyzed logs they have from other 777 flights in the region and noticed differences in the data depending on whether a flight was heading north or south. The MH370 data looked like the data from flights known to have been heading south. Hence their conclusion.