"ADS picks up the TAS, but if speed is unreliable, surely this read out could be unreliable to"
If I am not mistaken, it is GPS speed or GS that gets reported, and not TAS "velocity" word that is used, not airspeed. (there is a capability to transmit airspeed too, but that's another story)
http://adsb.tc.faa.gov/WG3_Meetings/...x_A%20Mods.pdf
upd. In this particular case - I don't know what is the source of ground speed value that gets received by the ADS-B Out equipment. I mean - it could be GPS receiver or IRS + sufficient data processing.