Thrikeen would only break the square-cube law if they were 'normal' preying mantis' scaled up in size in a perfectly proportional way (like if you blassted one with an anti-shrink or enbiggening ray). But I don't think that's the case as far as I know - I doubt we have a detailed breakdown of thrikeen physiology.Or the preying mantic men that break the square cube law.
In otherwords, the square-cube law is relevant when you're scaling something up and a thri-keen is not a 'scaled up' preying mantis.