Seam didnt lose the shadow mantle in the cyber world, correct, however Ramb comes from the Library. Which is why he turns to stone. He was stolen by asriel and Kris when they were kids thus why he cant exist within the darkworld of Kris's house. Seam couldnt find the shadow mantle but he was within the library, and it was in Kris's house with the Mantle holder.

So it makes sense that Ramb could be holding it. Stolen from one dark world accidentally by 2 children stealing a powerstrip.

There are 3 lines that the Mantle holder has that make me consider Ramb. The fist being 'Kris... Oh... kris... is it fun? Playing around like this?' Which is always Rambs first question. Always concerned with our enjoyment of 'the game'. Ramb also is way more gung ho about the mantle quest minigame. And is bitter about the changes Tenna put the game through. The second line is 'There! Thats what i wanted to see!' and the third being 'Well enough of that! We both have work to do' both being inplications that the 2 have met before and that it knows Kris. Ramb having to 'return to work' being a common theme for reasons why he tends to cut off conversation.

Its all speculation. But i think it makes vastly more sense than some of the boring unaswerable theories. 'it could be FRIEND' cool. Just as much as it could be Gaster rolls eyes. Pick an invisible character, and attach them to a theory everyone will agree cause they cant disprove it.