Yes, Comey’s judgment that Trump is a liar is subjective. But the point is that he repeatedly and brazenly called Trump a liar. This is Comey’s explanation for why he (supposedly) took notes only on Trump meetings; because Trump could not be trusted.
From the same article:
‘Then, Comey said he took meticulous notes after meetings with the President because he believed it was necessary to preserve the truth. In other words, he believed President Trump would lie about the conversations.
“I was honestly concerned he might lie about the nature of our meeting and so I thought it really important to document,” Comey said.’
"I was honestly concerned he might lie about the nature of our meeting and so I thought it really important to document," Comey said.'
Yowza - I'd count that one as "calling Trump a liar" even though it's couched in a "maybe" or "he might."
Comey is a disgrace and a piece of excrement.