Actually in Chinese, that word Sen, or jian (煎) mostly means fried but also boiled in herbal medicinal context.
In Japanese, the primary meaning is boiled, simmered but also roasted and parched (please correct me if I am wrong). Considering the context and how Baisao appears to have executed the infusion, it is probably supposed to mean simmered/ boiled in the tea context during his time period
Interestingly, most of the steeped tea used by Baisao and his predecessors in Japan were probably not steamed. It seems the steaming process was developed a little later, though during Baisao's lifetime. Baisao started out using tea from China. Apparently Baisao praised the steaming process.
This book seems to be a good reference:
https://uhpress.hawaii.edu/title/tea-of ... of-sencha/
Here is an informative excerpt:
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