It's a non-immigrant visa, because the law uses "immigrant" as a technical term that does not match the common usage. For international comparisons, it makes more sense to say that H-1B is an initial immigrant visa that grants you a temporary residence and work permit. Most countries expect you to immigrate and live several years as a temporary resident before you can apply for permanent residency.

My personal rule of thumb is that any non-citizen who becomes a tax resident due to hitting the six-month rule is an immigrant.