I don't understand how the location of a 377 foot tall tree could be kept secret. Wouldn't that type of thing be visible in satellite imagery at the very least?

It's not sticking straight up from the ground in Kansas. Hyperion has many siblings nearby and is on rocky terrain which conceals its overall height.

From Wikipedia:

"The exact location of Hyperion is nominally secret but is available via internet search.[12] However, in July 2022, the Redwood Park superintendent closed the entire area around the tree, citing "devastation of the habitat surrounding Hyperion" caused by visitors. Its base was trampled by the overuse and as a result ferns no longer grow around the tree.[13]

Measures to protect the Hyperion tree were officially implemented in 2022 when the National Park Service (NPS) closed public access to its location in Redwood National Park.[14][15] Anyone who gets too close could face up to six months in jail and a $5,000 maximum fine.[13][16][17]"

It isn't a secret. The location can easily be found if you Google it.

There are a lot of 300 foot trees in the general vicinity, so you'd need to actually measure to be precisely sure