1) Translators fucked up; "fish roe" isn't shirako/milt, which is the semen of the male fish (not the name for the glands, just what they contain, though since it's often just removed wholesale from dead male fish and packaged that way, since it's much less messy, people have taken to misunderstanding that the whole seminal vessel is called shirako/milt), but the fish *eggs*, what we also call caviar.
2) Shirako, or milt, is actually a commonly sold item in Japan, and can be found in izakayas, sushi bars, street stalls, and more, where it can be found served raw, steamed, fried, grilled, in tempura, or as part of chawanmushi, a savoury egg-and-seafood custard. You can also, obviously, purchase it raw at a market to take home, as he has done here.
3)They do eat fish roe/caviar in Japan, too, obviously; it's commonly used as a sushi topping, in fact (the nori-wrapped gunkanmaki or "battleship-style" rolls with the little red balls on top you typically see as a common sushi roll in various media, including emotes? The red orbs are fish roe/caviar), much like how shirako/milt is.