When the thermostat is open solid… there is not time for fluid to slowdown and actually get cooled in the radiator. It is flowing pretty fast with those gears/engine rpm.
Also
I used to think that too but it is not the case. It is a wives tail, and has been debunked several times here and elsewhere.
If the fluid is going slow through the core it is also going slow through the block, slow fresh cool fluid entering the block means slow cooling of the block.