The spec says

A grid container is sized using the rules of the formatting context in which it participates:

• As a block-level box in a block formatting context, it is sized like a block box that establishes a formatting context, with an auto inline size calculated as for in-flow block boxes.

This is fine for in-flow block boxes, but for floated boxes (or abspos) the grid container should shrinkwrap. I think perhaps it meant "non-replaced block boxes" here.