A third ADS-B dataset for TK6491 is available here at RadarBox24, it might require some sort of subscription or registration:
https://www.radarbox24.com/data/flig...6491#148232065
The speed data is less finely sampled than with FR24 or FlightAware but it does appear to agree that the plane was decelerating all the way down the path to the runway.
If they did have the power back to idle trying to slow down descending into freezing fog, wouldn't those Pratts still have plenty of anti-ice capability with normal pneumatics? On some ETOPS twins you have a minimum power setting for anti-ice with a single bleed source but it is normally not a player with everything working.