Zen a religious practice? Religion is not just poly- or mono-theism but, more generally, is response to transcendence. The transcendent need not be personified as gods or God. It is also conceived as the impersonal Tao, Brahman, Dharmakaya etc. Also, the eternal is present and zazen is attention to the present.
(ii) Poul Anderson imagined extraterrestrial religions. Could he have imagined an alien equivalent of Zen or, better than that, aliens with sufficient mental control that a practice like zazen was unnecessary for them? I think so -
The Ishtarian forebrain does not shut down while sleeping so Ishtarians
can direct their dreams, for example can dream the Triad, and some can
communicate dream visions through words, music and dance, whereas for a
human being the Triad remains only "'...a philosophical concept.' " (p.
-copied from here.
An alien brain might function in such a way that memories were accessed only when needed for practical or academic purposes in the present and did not generate incessant, involuntary and unnecessary thought processes distracting attention from the present.