SB in economics here and current business school student (I know, I know: burn the future MBA at the stake!).
Indeed, it is a valid criticism. The market cap/GDP measure is mismatched, since market cap theoretically reflects investors' expectation of future cash flows globally while GDP is a measure for only one country. Also, GDP is problematic for a bunch of reasons, so even if all companies were only operating in the United States, GDP would still only be a crude measure of economic output.
Plus, as you mentioned yourself mcap corresponds to the present value of all future cashflows and GDP to one years output. The only way this would make sense is you were comparing changes in GNP to changes in mcap.
Indeed, it is a valid criticism. The market cap/GDP measure is mismatched, since market cap theoretically reflects investors' expectation of future cash flows globally while GDP is a measure for only one country. Also, GDP is problematic for a bunch of reasons, so even if all companies were only operating in the United States, GDP would still only be a crude measure of economic output.