It’s since been revealed that Sony is causing the speedbump — an Epic Games rep confirmed to TechCrunch that Switch cross-play is supported with Xbox, PC, Mac, and mobile. But if you’ve tied your Epic Games account to a PSN name, then your play won’t be supported on the Switch.
Not to mention as this was all blowing up, Xbox tweeted out "Wanna play Fortnite" rubbing salt in the wound. If Microsoft was blocking as you said, why would they tweet out something? They obviously are taking a jab at Sony.
AGAIN answer this very simple question: How can Sony block a game they don't own on a machine they don't own?
Sony is indeed the one not wanting cross-platform, nobody is denying that. But what is blocked on Switch is blocked by Nintendo, what's blocked on Xbox is blocked by Microsoft and what's blocked on Playstation is blocked by Sony end of story.
Unless you are implying that Sony hacked the Switch servers and injected code that makes those accounts not work on the Switch?