This seat map is for what Seatguru is referring to as "Layout 2". These aircraft contain 3 classes of seating - "Polaris" (aka Business Class, or sometimes First Class on domestic routes) in rows 1-12, "Premium Plus" (aka Premium Economy) in rows 20-22, and Economy (rows 30+).
Economy is further broken into "Economy Plus" and standard "Economy" - these are technically not different classes of service, however the "Economy Plus" seats generally have more legroom and are available to United Frequent Flyers, or to others at a cost.
On the specific route you're looking at, United will NOT be selling all 3 classes of service, but instead only two - Polaris/First (depending on the route) and Economy, however in this case they have scheduled a plane that happens to have Premium Plus seating. Thus it is not possible to specifically purchase a Premium Plus seat on this flight, even thought the aircraft physically has such seating.
Historically for such a flight, these Premium Plus seats are generally treated as Economy Plus seating, and can generally only be selected by United Frequent Flyers. Recently United appears to be changing how these seats may be selected, and some travelers have reported not being able to select them even if they do have access to Economy Plus seating. Anyone sitting in these seats will receive standard Economy level of service (meals/drinks/etc), not those that would normally apply to Premium Plus.
Depending on how long until your flight it's possible that the seat map will change before the flight, if United instead decides to fly a different seat layout on this route - however in this case that's unlikely as the other layout on the 787-9 (what Seatguru calls 'Layout 1') has far less Polaris class seats available. If that was to happen, these seats would become standard Economy Plus seats, as per the other seating layout.