The NEC doesn't say actually say that.

However this wire that's attached to the device by it's manufacturer isn't subject to the code. It's all ready been approved by its NRTL (typically UL). First, these wires have different ampacity rules than branch circuit wiring. Second, they are sized to the actual load. In this arrangement the #10 wire is probably good for 28-30A (typical for a wall oven). The branch circuit however is sized so that the load is only 80% of it's capacity.

If you have manufacturer's requirements you're required to follow them. If the manufacturer only terms it a recommendation, you'd still be best to follow it.

Thank you ronnatalie, I understand the derating of the circuit for "continuous" use but why would the 6' flex whip not be a part of the branch circuit? The electrons don't know that they there is a jbox in the wall? Also the maximum load when both micro & oven are on is 33 amps. So how does U.L. allow that to pass?

Often those integral whips have insulation that is rated for higher temperatures than is the typical conductor used in residential wiring. The wiring in your residence is rated for maximum temperatures that do not cause significant degrade of the insulation. Plus the appliance is expected to have a much shorter life than the house in which it is installed.