Their ruling wasn't about the impeachments, it was about whether or not Trump could be held criminally liable as a former President. Trump's team tried to make the argument that he couldn't be held accountable for actions taken while in office because he wasn't successfully impeached first, and the panel ruled that not only was he NOT "in office" when the offense took place, but as you noted the impeachment proceedings were not a legal trial and were of no concern to any court issue... but they still managed to get in a jab about the real reasons why he wasn't impeached despite a mountain of legal evidence.