Seeing Astrid emerge from the Space Gate portal provided Kethra with the much-needed support and confidence to confront Alec and his group of golems.
It reassured him that he had a backup plan if things didn't go as planned, similar to the first time when things went sideways, and he was able to escape.
However, Kethra wasn't about to let Astrid see how grateful he was for his unexpected arrival.
After all, Astrid had never mentioned anything about providing this assistance in the first place.
It made Kethra suspect that Astrid had planned to catch him off guard from the beginning.
This realization made Kethra grateful that he hadn't obtained the attributed stone yet, or it could have been used against him.
Since Astrid possessed such a powerful artifact, it would make it Much easier for him to track him if he were to escape during a fight.