When I was first learning about fiber, graded-index multimode was the "hot new thing" with corning promising the modal-dispersion of single-mode fiber with the light-carrying capacity of multimode, which should reduce repeaters compared to either. Since these are single-mode fibers, I assume those promises were overstated?

Yes, and the SDM as described in the nature article in parent^2, it would require a something far more complex than a repeater (which in most cases is actually just a purely optical amplifier).

Current practice is to use erbium doped fiber amplifier or raman amp for boosting optical signal at long intervals for transoceanic runs. Given the complexity of spatial signal, I don't think a regular optical amplifier will work? I could be wrong, this tech is changing but submarine fiber-optics tech is necessarily conservative and slow moving.