3-prong vs 4-prong
3-prong connections lack ground; they bootleg ground off neutral and if neutral breaks, it guarantees the appliance is electrified.
Appliance manufactures are terrified of losing sales (or much worse, getting customer returns!) because the installed power was not right for their appliance. That's why they lobbied to allow new 3-wire bootlegs so late (1989 they were finally outlawed?) and to have old 3-wire connections stay grandfathered. Hence shipping it with a 3-wire, and the sneaky language encouraging 3-wire even though that's usually illegal and certainly unsafe.*
If your wall box doesn't have separate neutral and ground, then either retrofit a #10 ground wire (allowed now), or put a GFCI breaker on it and label the socket "GFCI Protected/No Equipment Ground".
40A vs 50A
Common kitchen range circuits are either 40A or 50A.
If it's wired with #8 cable, it must be breakered for 40A. If it is wired with #6 cable, it can be breakered 50A - though a 40A breaker is also allowed. (that would be a case of using larger wire than is required; that's always allowed).
Either way, they use a 50A receptacle because 40A receptacles do not exist.
This manufacturer chose to pay a little extra to Underwriter's Laboratories (UL) to certify their oven both for a 40A supply and for a 50A supply. So the customer can just "drop it in" to either situation. It's back to avoiding lost sales.
That is legal because UL says it is legal, because it passed UL's battery of tests and design requirements. Given the narrow ampacity range, that's not a high bar.
* what they fail to mention is it's required in most cases. You're not allowed to downgrade a 4-wire connection to 3-wire (the sneaky instructions sure sound like they're suggesting that, eh?) -- So if the house was built 4-wire (certainly any after the 1989? NEC came to force)... or was previously upgraded for 4-wire, you can't downgrade it now. Metal conduit installations, conduit is the ground. "Local codes" (read: they adopted NEC) do not permit 3-wire in any of those cases, and that's effectively national. Add to it localities who put their foot down and said "ENOUGH!"
In short, if neutral and ground are separate in your box, you must use 4-wire.