Fluctuations in the mass of the pie at shorter intervals would appear to observers within our universe as discontinuous jumps between discrete quantities of pie. Therefore, during that interval, the pie is fixed.~

Now go away.~

On a geologic timescale, pie has just appeared out of nothing, and is now covering the entire planet. On an infinite timescale, by naive extrapolation, the pie will in the future be expanding faster than the universe itself, to the point where a wafer-thin bite of pie will expand and rupture the esophagus before peristalsis can even push it into the stomach, and persons dying from attempted pie consumption will literally explode in a shower of pie.~

Clearly, the parent post was substituting a hyperbolic term for the longest possible economic timescale, where new technologies may be invented and entirely new supply chains built based upon them. It is easy to claim that at that scale, economic growth will continue without bound for as long as human ingenuity can conceive new ideas.

At shorter scales, the observable size of the pie does sometimes shrink. And if there are periods when it grows, and periods when it shrinks, then logically, there must be periods when it remains the same size, even if those periods are very short.