That is not however the postulate that compares to the canonical statement “rūpa is not the ātman” - where as per the statement you’re comparing, it would be you yourself (not someone else) making the original statement that ‘this shape is not a square triangle’ (so you utter the initial statement of absurdity) and then you yourself also make the claim that the square triangle is unreal i.e. you also assert that a square triangle is an absurd conception and hence never had the possibility of being real. Therefore having the referent reduced to a void, and to then say “this shape is not a square triangle” is the statement of an absurdity, and devoid of a referential integrity, turns out to not contain any factual sense. The first sentence is therefore firstly an inanity, and a hollow vacuous falsehood.
Even if you want to have the original statement the way you want it, i.e. a positive assertion of absurdity “this shape is a square triangle”, you would be the same person making it and simultaneously claiming its unreality and absurdity. For you are the one that was defending the simultaneous accuracy of both statements. Now you are pretending the original absurdity was uttered by someone else.
The statement “this shape is not a square triangle” is as absurd as the statement “this shape is a square triangle”, because you are theorizing with an absurdity as an integral part of the statement of the theory (by giving that unreal thing a conventional reality to make the sentence appear semantically valid). Whether you say “is” or “is not” doesnt make the statement meaningless, it is the use of an absurdity in one side of the equation that makes the whole sentence meaningless.
Thus it turns out you are making mutually contradicting statements (by even pretending that it is truly possible to define something real by excluding from it a void - like attempting to take-away/subtract a non-number from a number, or vice versa).
In an excel spreadsheet, put in a formula where you reference cells in other worksheets within the same workbook, then delete the sheets that the formula is trying to refer to, and see whether you get a referential integrity error or not. Try convincing Excel that it must not report any error (and still process the rest of the formula as if the missing reference never existed in the first place) when you delete the reference cells that you originally relied on as being real when you wrote the formula. Or better yet, in the formula try to refer to a non-existing cell and see if the formula works, such as A1=B1-(imaginary cell).