OED:
candour - the quality of being open and honest in expression.
"They didn't state he lied ... without even using lies of omission ... they said he wasn't [word defined as honest and open]"
Candour encapsulates exactly those things. Being open (i.e. not omitting things and disclosing all you know) and honest (being truthful).
On the contrary, "not consistently candid", while you call it a "generalized term", is actually a quite specific term that was expressly chosen, and says, "we have had multiple instances where he has not been open with us, or not been honest with us, or both".
If he didn't lie, and didn't lie by omission then he was by definition being candid.
Give us an example then, of how you can be "not candid" while being honest and open.
If I take the time to accuse my boss of failing to be candid instead of thanking him in my resignation letter or exit interview, I'm not saying I think he could have communicated better, I'm saying he's a damned liar, and my letter isn't sent for the public to speculate on.
Whether the board were justified in concluding Sam was untrustworthy is another question, but they've been willing to burn quite a lot of reputation on signalling that.
Business communication is never, ever forthright. These people cannot be blunt to the public even if their life depended on it. Reading between the lines is practically a requirement.
It is even possible to not be candid without even using lies of omission. For a CEO this could be as simple as just moving fast and not taking the time to report on major initiatives to the board.