While the engine certainly sees much higher forces as the RPM are increased, transmission is also subject to increased forces when the engine RPM is increased. Ford uses the 6R80 transmission in other vehicles, including the F-150 with several engines. In that appliocation Ford says:
"Ford says the 6R80 has been through 150,000 equivalent miles of testing cycles that reflect the most extreme-use customers, and the box itself is rated up to 7,000 rpm and the fluid is filled for life."
See: Compact Equipment Online - Purpose Driven
So it isn't only the engine that can be subjected to extreme forces when the engine RPM begin to exceed the red-line.