However if they are left in FLEX without a flex power computation I don't know what it would go to.
The detent is FLX/MCT.
FLEX has arming and engaging phases. When the thrust levers are moved to FLX/MCT, but the FLEX conditions are not met, then FLEX mode will fail to engage.
That means the engines will stay at MCT, which is not enough for takeoff.
Hence the "retard, retard, retard" warning at 80 kts, which the pilots elected to ignore.