I noticed that too. I like the idea of him covertly finding about Jasnah's abilities. If what Kabsal said was true, a normal Soulcasting (using the fabrial) involves tapping on the stones, then that stranger was doing it without a Soulcaster as well. Also, the gemstone is on the man's palm, and no mention is made of the rings, bracelet, other gems, or fact that they usually go along the back of the hand. Unless Jasnah has been wearing it backwards this whole time by accident?
I'm sure he didn't want to reveal what he and his people were capable of when Jasnah was around, but it's also possible that he just didn't have any of his people nearby.
Just to be clear, you can't Soulcast without gemstones, Jasnah needs the right type of stones infused with Stormlight to do any Soulcasting. When she Soulcast that rock, she drained her Smokestone, when she killed the thieves, she cracked the Smokestone and drained the diamond and ruby. She also needed a garnet to Soulcast Shallan's blood near the end.
Now here's my question, is Tavarangian allied with the ghostbloods? Or a separate organization all on his own?