This was told by the head of Epic Games, Tim Sweeney, during a hearing in the case of Epic Games and Google due to the removal of Fortnite from Google Play.

Sweeney admitted that in June 2018 he sent a letter to the vice president of Sony Interactive Entertainment, Phil Rosenberg, in which he stated that sooner or later all platforms will begin to support the crossplay feature in Fortnite. The company was ready to fight for crossplay in all possible ways, "wherever it leads and no matter how long it takes."

Eventually Epic Games and Sony settled their differences, and in September 2018 Fortnite received a crossplay on PlayStation.

Note: during the trial of Epic Games and Apple, it was reported that in 2018, Fortnite developers also offered Sony various bonuses for agreeing to support crossplay. In particular, Epic Games promised to make an exclusive for PlayStation VR 2, make concessions in the license for the Unreal Engine 4 engine and add a unique character to Fortnite for PlayStation Plus subscribers.

At the same time, Sweeney called Sony the only platformer that requires paying a commission for crossplay with its console.

A source:

The Verge