"By state law, the city can force developers to include affordable housing in developments only if it comes in conjunction with an upzone."

RCW 36.70A.540(1)(a): ... 
An affordable housing incentive program may include, but is not limited to, one or more of the following:
(i) Density bonuses within the urban growth area;
(ii) Height and bulk bonuses;
(iii) Fee waivers or exemptions;
(iv) Parking reductions; or
(v) Expedited permitting.

Where does it say "only ...an upzone"? In fact, it's very difficult to read (1)(c) without concluding the city cannot "force" the developer to participate: "If a developer chooses not to participate in an optional affordable housing incentive program adopted and authorized under this section, a city, county, or town may not condition, deny, or delay the issuance of a permit or development approval that is consistent with zoning and development standards on the subject property absent incentive provisions of this program."

Can someone explain?