This (from TPM) is true.
Let's remember that it was Shinseki, as Chief of Staff of the Army, who publicly stated in congressional testimony that the amount of resources the Bush administration was committing to the imminent invasion of Iraq was vastly less than would be required - largely in terms of the number of troops required to sustain any sort of occupation.
Paul Wolfowitz then publicly dismissed Shinseki's testimony and after that his influence and clout went into rapid decline. Whether the actual timing of his retirement was tied to this remains murky. But there's no doubt that this stand was basically a career-ender. And much of the history of the subsequent occupation of Iraq was a slow motion vindication of Shinseki's warnings.
The point isn't that Shinseki was right. A lot of people saw this at the time. And generally in our public life there's too much score-keeping of who was right or wrong about this or that thing that's already done and done with. To me the point is that he spoke up at real cost to himself.
None of that means he should get a pass for whatever happened under his management of the VA. But it shouldn't be forgotten either.