The guy was obviously guessing about the capacity of the other aircraft because he didn't know which one it was.
He was even guessing about the capacity of his own aircraft, because the "289" number he uses, as others have pointed out here, isn't any configuration of seats that United flies in the 757-300. I bet the 289 number comes from the "757" Wikipedia article, which states, about the -300 variant:
"Eight standard cabin doors, plus a pair of over-the-wing emergency exits on each side, enable the 757-300 to have a maximum certified capacity of 289 passengers.
It seems that's where the 289 number comes from. Yet he uses it, and then multiplies it by 2, and asserts on the basis of this total that this incident would have been
the biggest air disaster ever - by something like 8 or 10 souls over Tenerife. Without inflating both aircraft capacities to this unsupported high number, he wouldn't have been able to make that statement, which is the emotional lynchpin of his argument - indeed, the headline of his article (and therefore, I assume, his clickbait.) This was a misrepresentation, whether unintentional (and sloppy), or intentional (and therefore deceptive.)
As I commented down in that cbslocal.com article linked by RonR, this doesn't change the seriousness of a possible midair, no matter where it happens ("out over the ocean" just adds a scary emotional element.) But since the author's statements of opinion (like the system is "broken") are based upon his presentation and interpretation of apparently objective technical facts, it leaves me feeling like if he can't get his facts right, and possibly even inflates and distorts them to make his case, then it severely undermines his opinions, which I now assume are not anchored to reality.
Edited to add: Just found a great posting deconstructing the incident and Mr. Townsend's essay about it. This aligns with my opinions, for what they're worth...http://www.nycaviation.com/2014/05/near-miss-near-hawaii-didnt-happen/#.U4A00Haz58E