～というのは事実だ【というのはじじつだ】

というのは事実だ means "it's true that", "it's a fact that": the clause before it is presented as a claim, and じじつ事実だ confirms it. It follows a plain-form sentence, and a noun or な-adjective needs だ before という: ×優秀というのは事実だ.

Most often the fact is granted before a "but": the speaker admits it and then says why it does not settle the matter. が or けど follows 事実だ.

じじつ事実ではない says a claim is untrue, often a rumour or an accusation. じじつ事実だった says a claim turned out to be true. のはじじつ事実だ without いう means the same; という frames the clause as something said or claimed before confirming it.