Am I wrong?

You are correct: the function is there but it's not exported:

func isLeap(year int) bool {
	return year%4 == 0 && (year%100 != 0 || year%400 == 0)
}

I'm not sure it would be worth it to make the time API bigger (it is already quite big) for a function this simple to write.

I've tagged this as a proposal, leaving for others to decide.

There's a cost (maintenance, documentation, cognitive load from doc/API bloat) to exporting unnecessary or uncommon things.

The proposal process and gathering the data to justify why this should be exported and showing how many people would benefit will be more work than just doing the exercism exercise.

I doubt many people would need IsLeap.

You can just copy it (as @ALTree showed above). Even if you didn't know the leap year rules, you could use the time package to extract the information too:

func IsLeap(year int) bool {
	lastDayFeb := time.Date(year, time.February, 28, 0, 0, 0, 0, time.UTC)
	nextDay := lastDayFeb.AddDate(0, 0, 1)
	return nextDay.Month() == time.February
}

(but that's less efficient than just year%4 == 0 && (year%100 != 0 || year%400 == 0))

Thanks for replies, I get your point.

And it's true I've probably used less than 10 times isLeap method on professional projects for at least 10 years in other languages. So we are letting that cost to projects that require isLeap function, I'm closing this issue!