Formal poetry, as a craft, requires the use of a scheme to create a work, usually one long accepted or a recognizable variation on one of those "traditional" schemes (as most of those long accepted schemes, such as the Shakesperian sonnet, once were themselves).
Does this mean that conceptual poems are also formal? They require the poet to follow a scheme. That scheme is critical to the effect of the poem (indeed, in the pure conceptualism discussed
here, the scheme is more important than the poem itself). And that scheme is repeatable by others to different effects, which may be the scheme's most necessary formal quality.