The device only signals that it's a power sink with its 5.1k resistors, there's only one way to put them there without implementing the full-blown PD protocol. It can't signal power levels (USB legacy / 7.5W / 15W) and doesn't need to, it's the power source that signals its capability.
You're probably confusing it with the spec requiring that the device does not attempt to consume more power than USB legacy from a source that doesn't advertise such capability - but even then there's no "negotiation" happening, it's just a matter of measuring the voltage on CC lines.