It’s unnumbered regardless of outlined.
Why not let theorem = theorem-box then?
If I have to guess what you want, then you can override the title.
#import "@preview/theorion:0.3.3": *
#show: show-theorion
#let theorem = theorem-box.with(get-full-title: (_, title) => [Theorem (#title)])
#theorem-box(title: "Theorem without numbering")[
This theorem is not numbered.
]
#theorem(title: "Theorem without numbering")[
This theorem *is* numbered.
]

Which I guess is not documented.
Cc @OrangeX4