"Perhaps some familiar faces still reside on the Plane, remembering him after all this time . . . or even carrying on his family name."
That means Satoru may simply be someone who borrowed Umezawa's name to reform the Hyozan Reckoners, consider Toshiro's true heir is on Dominaria.
*cough* Ray Skywalker.
Or Satoru can easily be a descendant of one of Toshiro's relatives.
It was mentioned in the first Weekly MTG stream about Neon Dynasty that it's a bit of both. Toshiro still has some descendants on Kamigawa, but there are also some people who just adopt the name. Time (and the story starting on January 24) will tell which one Satoru Umezawa falls under.