At the end of Ep 8, we see Ultron gaining access to the multiverse and thereby goes on destroying them. Assuming Watcher took at least some time, Ultron could've easily traveled to a few different universes. This is confirmed by the line, if I'm not wrong (I could be), "I destroyed galaxies with my mind, why dont you just die" implying he traveled to at least one other universe as his was already destroyed to the core. If the Infinity Stones are what gives him his power to destroy, why couldn't he have taken the infinity stones from the other universes. The fact that there are infinity stones is confirmed by Gamora saying "The Crusher was designed to destroy the stones on my world, not his"