So what half truths was he correct on?
"Half truths" mean "statements that conveys only part of the truth, especially one used deliberately in order to deceive someone."
So, in answer to your question:
Before the inauguration, Pieczenik said that the troops were in Washington DC in order to mount a coup so that Trump would remain President. He took the fact that there actually were 25,000 troops in Washington, as well as the fact that there were more troops in DC than in Afghanistan and Iraq, to create a false narrative that they were there to keep Trump as President on January 20th instead of to inaugurate Biden.
Pieczenik predicted that Bolton and a few others would resign or be fired, which was predictable because so many cabinet members resigned or were fired, but would claim it was because he had special insider knowledge of the Trump campaign. He would then use those "true" predictions as "credibility" or "cachet" when trying to convince people of false narratives like, for example, his "inside information" that Hillary would be arrested.
I take it you are unfamiliar with Pieczenik?