Thanks Wrench,
From the link you provided:
Subtypes 1 and 2 are used to report ground speeds of aircraft. Subtypes 3 and 4 are used to report aircraft true airspeed or indicated airspeed. Reporting of airspeed in ADS-B only occurs when aircraft position cannot be determined based on the GNSS system. In the real world, subtype 3 messages are very rare.
Sub-type 2 and 4 are designed for supersonic aircraft. Their message structures are identical to subtypes 1 and 3, but with the speed resolution of 4 kt instead of 1 kt. However, since there are no operational supersonic airliners currently, there is no ADS-B airborne velocity message with sub-type 2 and 4 at this moment.
1
These messages contain more information than just horizontal and vertical velocity. Two other significant types of information are the navigation uncertainty category for velocity, and the difference between the GNSS height and barometric altitude.
GNSS height and baro altitude were reported, and the aircraft position based upon GNSS system was known and reported, and it was not a supersonic aircraft. So, from my inference of the information above, sub-type 2 & 4 speed information would not have been reported, as it was not supersonic, and sub-type 3 speed information would not have been reported, as the GNSS position was known and being reported. So we're left with sub-type 1 information, which is a GNSS derived "groundspeed" (IAS/TAS not being reported at all). Do I have it right so far? And, vertical speed (down) was reported, I presume from the GNSS position.
What remains unclear to me, is the "groundspeed" being reported, two dimensional, parallel to the surface of the earth? Or three dimensional over the earth? If the aircraft is in a very steep dive, the two dimensional groundspeed value would be artificially slow, and no longer useful information in the context of "did the plane exceed Vne?" If the ADS-B GNSS based "groundspeed" is actually a speed based upon a three dimensional calculation, it probably equates to IAS, at least roughly, and now has value in understanding how the airplane saw the speed relative to it's limitations.
The last sentence of the linked article says:
It is possible that with strong tail wind, the ground speed of an aircraft may appear to be “supersonic” during the flight.
Which makes me think that in two dimensions, ADS-B reported "groundspeed" and IAS could be quite different, but I don't know how to infer a vertical speed component in that.