To my knowledge, it was “highly unlikely” under normal circumstances they could have locked on a SR-71. But if they tweaked their radar properly, and I worked with the Swedes and they were/are pretty sharp, I will say it was possible.
I don’t entirely discount it either. According to the write-up in Wikipedia the Swedes made the intercepts on the return leg and fed the ground-base radar track directly into the Viggen’s fire control computer. That’s how they beat the jamming from the Blackbird. Score 1 for the Swedish radar geeks.