What do you call "internal oscillator"
Is it an oscillator built in the SF3301, or does it refer to the on board oscillator?
Is one on the PCB not visible on the right of their board picture?
yes, and it's also on the schematics, but even if you have an internal oscillator, It's quite common to use an external one for some reasons, so the "internal" word may be a translation mistake referring to the one on board as opposed to the one from another daisy chained board, the signal being transferred by the coaxial cable (RF receptacle).
I think that they mean internal circurity necessary for crystal rezonator to be working (negator/amplifier). You may connect crystal rezonator or "CMOS level" clock signal, pin OSC_BP is responsible for selecting propretiary mode. That's my guess.