Oh hey, another Trumper is coming up with weird shit around truths. This time is was Giuliani saying “truth isn’t truth” in an interview regarding Trump talking to the Mueller investigation.
That is literally Giuliani's role in this administration: to convince Trump's base to side with Trump over the rest of the country when and if something finally emerges that proves beyond a reasonable doubt that Trump is a fraud and/or traitor. That may never happen, but between Trump's conviction that this is an FBI blackmail operation and the vast array of shady things he's known to have done in the course of his career, one can see how they'd operate under the assumption that something will emerge that throws his core narrative into disarray.
In that context, Giuliani is a perfect choice in part because he's a blithering idiot; every shambolic account of the Trump administration he rambles through is just a shade more ridiculous than the last so as many of the base as possible accepts it rather than make the painful admission they've been conned by a cabal of morons. Every new meme lets the bar slip a bit lower. It's not dissimilar to how some criminal organizations secure the loyalty of their low-level members through progressively more heinous crimes: if they walk away from the murder, it means the assault was all for nothing, and so on.
So it's not, in a literal sense, "truth isn't truth." It's "Either truth isn't truth, or the liberals were right about you." It will be something weirder next week. All so that if the truth and Trump should ever diverge so sharply and so critically that to believe in reality is to demand his imprisonment, they can proudly choose not to.