Well, let's say that it's as you say, although your point will have better weight if you prove how the archaism won't play a part in the similarities among the languages of a certain family language group.
And Thracian was certainly a part of the Balto-Slavic family group at one point in history.
So, if the Slavic and the Baltic languages split off around LBA-EIA, then the Thracian and the Baltic would have had split off at a later date.
Still, lot more connected with the Slavic language group than with any other Balkan language group.