I just checked both, and no external interface is needed or desired with the FT-900 and AL-811H.
Just a waste of more stuff.
When Ameritron recommends the ARB-704 or whatever, that's a "general" recommendation in case a user wants to connect a modern rig to an older amplifier that uses either higher voltage or higher current keying. The AL-811H doesn't; it's 12Vdc at 100 mA, and the FT-900 can key that just fine all by itself.
I'd double my money by folding it in half and putting it back in my pocket.
What if soy milk is just regular milk introducing itself in Spanish?