But did he travel extensively enough?
Put yourself in his shoes (or spider chair thingy). You're the last of your race. You've been away a long time, come home and find everyone gone. Wouldn't you travel far and wide searching for others from your family, community or very species? Wouldn't your loneliness and isolation be unbearable? I'm sure he left no stone unturned anywhere in all the worlds he knew how to visit. He's the very last Deep Elf.
And if the City was a Dwemer city and away from Nirn where the Dwemer disapear,whos to say they didn't become incredibly advanced?
Keep in mind that the Dwarves didn't invent technology themselves. The beings called gods and ancestors by mortals on Tamriel came from other worlds, and landed in an honest to goodness space ship at one point. The Dwemer being Elves were descendants from the children of Aldmeris, as were the Chimer who became Dunmer. Perhaps Sotha Sil didn't copy the Dwarves but instead both he and they copied the same original architecture.