The C-band (NATO designation, not IEEE) portion of the Monolit system could be OTH capable, though not through either skywave or groundwave backscatter. Open-source material strongly suggests the upper frequency limit of skywave propagation is 30 MHz (lower A band), with the groundwave propagation limit even lower.
However, UHF radio waves (which the C band is a part of), and to a lesser extent VHF, are subject to range-extending tropospheric propagation, which happens best over water and flat land. The bad news is that is dependent on the weather (warm, calm and high pressure are best), time of day (or more specifically, nighttime), and season (most pronounced in summer/autumn). This also should apply to ESM receivers that pick up UHF signals.
This doesn't apply to the X-band (NATO I/J) portion of the Monolit; the frequency is too high for any meaningful sort of refraction beyond the horizon.
Thanks! I did some related reading on this and stand corrected. We'll need to update the OTH flags of these two emitters in the DB.
So, we agree that (as long as there is no land mass between them) the C-band element can indeed pick up the Visby.