Because a giga-coaster is specifically defined as smaller than that one:

"Enthusiasts use the term to apply to a roller coaster with a drop of 300 to 399 feet, meaning that its riders fall the length of a football field. (Anything beyond 400 feet and you’re in strata coaster terrain.)"

So yeah, weird article.

A giga coaster is between 300-399 feet, while a strata coaster is 400+. I guess they didn't include the two strata coasters (Top Thrill Dragster being the second) because they don't go through as long of a ride.

https://en.wikipedia.org/wiki/Roller_coaster#By_height