The name "20th Ave NE" seems correct to me. The street sign where the trail intersects NE 98th St says 20th Ave NE, and I'm not aware of any other name for it.

It's not abnormal for trails and staircases to have road-like names. For example, the intersection of 20th Ave NE (road) and NE 95th St (path) has a street sign that says NE 95th St despite NE 95th St not being a road at that location.