The headline is substantively misleading; Google isn't planning to boost Amazon's competitors, it is planning on extending the degree to which it is Amazon’s competitor (which it already is, in a number of spaces, including product search and retail marketplace) by extending it's product search / retail marketplace strength.
Now, unlike Amazon, Google's marketplace approach (both the existing approach with Google Express and the approach with the new offering which seems to largely involve the same partners at launch as Google Express already has) keeps the actual retailer's identity front and center, and facilitates retailer-consumer ongoing relationships as opposed to Amazon's approach which minimizes seller identity.
Beyond this, if Amazon was willing to participate, I'm pretty sure Google would let them in. However, knowing the companies from prior actions, there's no way in hell Bezos would ever allow that to happen.
Now, unlike Amazon, Google's marketplace approach (both the existing approach with Google Express and the approach with the new offering which seems to largely involve the same partners at launch as Google Express already has) keeps the actual retailer's identity front and center, and facilitates retailer-consumer ongoing relationships as opposed to Amazon's approach which minimizes seller identity.