Fallacy when serial liar says, "I'm only human. We all tell lies"?

That behavior is called minimizing. Liars and abusers try to minimize how bad their behavior is when confronted, and one way to do that is by telling that they didn't do anything worse than anybody else does. They try to appear like it's something normal.

As a fallacy it's more close to false equivalence .

For the second argument, it is also strawman fallacy , because the accusation was of deliberately trying to deceive, not just writing something that is not 100% accurate.

(An also most common response liars and abusers may use to excuse themselves is to say "no one's perfect" when they are confronted about something reprehensible that they did. That's also strawman fallacy (substituting "abusive" with "imperfect"), and also an attempt to minimize such behavior and gaslight the victim.

#1: Suppose someone is demonstrably a serial liar — they lie about pretty much everything. When confronted with this fact, they respond,  "I'm only human. We all tell lies."

#2: Now, let's say the serial liar writes an article full of fabrications. When confronted with this fact, they say, "If you look at any article close enough, you're bound to find errors."

I think the reasoning relies on equivocation. Yes, virtually all humans lie about some things - even saints. Sometimes we lie out of embarrassment, to protect someone/something, or to be polite. Arguably, the aforementioned kind of lies can be constructive - hence the term, 'white lies'.

There is a stark difference between that, however, and the destructive lies told by a pathological/serial liar, both in terms of how severe the lies are, and how frequent they are. A regular human being, even though they may lie on occasion, can still be trusted to tell the truth most of the time. A serial liar, on the other hand, cannot be trusted at all, because they are willing to bend the truth far more than your average person.