Is Halo: Infinite actually Halo 6?

I haven’t read anything official that explicitly said that it was.

Yes. They confirmed that it takes place after halo 5. It really is halo 6 just not called it. 343 said they want to move away from numbered titles a few years ago

Yes. They said it is the continuing adventures of the Chief after the events of Halo 5. They have moved away from numbering (they tried to ween us off number in the titles with Halo 5 Guardians). This is the next mainline FPS Halo game and continues the Reclaimer Saga.