I don't think this argument holds up. If I cut 24 slots in a fretboard but then plug those up with a material harder than the original wood itself, you're telling me it'll cause the neck to bend/bow more easily? Especially seeing as the fretboard accounts for maybe 1/5 the thickness of the neck, this seems suspect.
If they said something like "the composite/carbon fiber fretboards we use don't take slots well" then I could get on board, but "this is stronger" just seems dumb.