You're using gtsummary version 2.1.0.9010 and the theme element "tbl_summary-arg:type" was not yet supported in that version. It was introduced in version 2.2.0.

And you need to explicitly say that you don't want dichotomous-style summary:

reset_gtsummary_theme()

trial |>

tbl_summary(

type = all_dichotomous() ~ "categorical",

dichotomous = "no"

) |>

as_flex_table()