The article is talking about 4 year olds (and, yes, 3 year olds, too).
While the article often uses the term preschool, the law is for "child care centers." At age 3 (and often at 4) the institution might be called "preschool" but tends to look and function like daycare. I've seen "daycares" that start worksheets at age 3 (eww). The lines btwn daycare/preschool aren't always crisp, but effects of suspension are.