I have a package of Harney & Sons "Japanese Sencha Premium Green Tea"
bags.

The brewing instructions on the box clearly call for boiled water to
be poured into a warmed pot or mug (containing the tea bag) for three
minutes.

I woud imagine Sencha would (should?) taste better when brewed using
water from the kettle at 170 or so (maybe even in an unwarmed cup?)

What's going on?