The flywheel is cast iron the gear on your starter motor is definately not.
I am just a mug repairer not a B & S certified master technician.
There are a few of them on here but as far as I know there is no iron starter gear and the steel gear is only used on Vanguard engines but as always I am happy to be proven wrong, that is one way we learn.
I have stuffed up enough starter gears to recognise a gear lock when I see one which is why I suggested you test the meshing of the gears.
There are a few other things that will stop the engine dead like that, but all of them will also stop the engine when you try to rotate it by hand as well.