Well, those are rumors assuming a chiplet architecture, which Apple has never tried before and would require very significant modifications to the layout.
Simply quadrupling the die isn't really feasible, the price increases exponentially.
Well analysts have estimated the cost of the M1 at $40 to $50 per chip, so even if they double the size of the chip which quadruples the cost, that would still be completely feasible for the Mac Pro which retails at c$5,000+
Even if the SOC was $500 cost price they would still have plenty of room to pay for everything else and hit the $5k price point.
I mean, they could even use separate highly integrated chips to get that number of cores too, although I suspect they will want to do it on a single chip assuming cooling is OK.
Simply quadrupling the die isn't really feasible, the price increases exponentially.