While the vertical integration is definitely the best way to get it done, it's not strictly required as long as there is good enough documentation for a platform. Linux originally supported Intel without any Intel engineers even knowing it existed.

Also consider Apple's chips, which have gotten Linux support without Apple ever submitting a single line of code.

While Qualcomm's behaviour is definitely a massive bummer (not to mention Qualcomm's competitors), it doesn't stop manufacturers from supporting their devices. It merely stops maintaining support from being cheap and easy.