Big Max - Vendor: Twilley Seeds. Characteristics: up to 100 lb; pinkish-orange rind with bright yellow flesh 3-4" thick; good cooking quality. Twilley Catalog, 1989.

Which indicates it's a rather recent introduction but doesn't say if it's heirloom or not.

Heirloom seed companies list many varieties that aren't necessarily heirlooms.

In other words they are open pollinated but not documented to be heirlooms based in date of existance, which for many folks is about pre-1940.

So all heirlooms are open pollinated but not all OP's are heirlooms.

I've never seen Big Max referred to as an heirloom.

Carolyn