I believe Typst decides math.class for every operator, and triangle.stroked.t appears to be "binary" by default.
Follow this other solution to alter math.class of a symbol:
So, for example:
#set text(font: "New Computer Modern")
#show math.equation: set text(font: "New Computer Modern Math")
#show sym.triangle: math.class.with("unary")
Let $A triangle B$.