Best way to distinguish between a light Poplar and Maple is that Poplar will dent easier than the Maple.
True, But we could not touch the wood in the pictures to try to dent it. But physical testing IS part fo actual identification.
As far as knots, clear pine doesn't have them nor sap pockets. It's very common on the west coast.
That piece could simply be taken to a local lumberyard for identification. And it may very well be poplar (not "popular").