From what he says and from the karma going up the account seems legit
We can't expect him "to tweet his HN-Name" for verification. He could, though I expect him to have strong opinions on things in general (leading to quickly reject a lot of (wrong) paths)
My point was, the account was taken by someone a day ago. And I am pretty sure Jan Koum would not be commenting on HN article at the time of a massive outage in WhatsApp. It may be him, but very unlikely. My 2 cents.
Interestingly, their Status Twitter account says "We are working very hard to make this twitter account irrelevant.". As it has not been updated at all today, I'd say they have accomplished it.
