Then obviously his letter could've said as much. It didn't, ergo an interpretation of available evidence clearly indicates you are wrong.
If he was in an open relationship or had a girlfriend while separated from his wife, that's not really something to be embarrassed about. His letter talks is him apologizing to his family and asking for forgiveness. He got caught in a Chinese intelligence honeypot and knows he fucked up.
It's clearly the CN link and/or the cosying up with a customer that is the problem; the board would not care about his private life (and neither should the public, tbh).
And the open letter sounds more like it was written the way the public would expect a CEO to write.