"Demand for the Chinese tech company's [Huawei] devices is red hot even though the country's overall market for smartphones is shrinking."

That's the second sentence in your link. And it says Oppo grew at 1.1% (in 2018q4)-- that's not exactly winning. Xiaomi plummeted 28%, even more than apple. There's clearly more going on than a simple competition between apple and Huawei.

https://www.engadget.com/2019/03/13/smartphone-sales-china-p...

More recent data shows Oppo and Vivo are struggling to increase sales too. Huawei is the only company growing in the Chinese market. 2019q1 data:

https://www.canalys.com/newsroom/canalys-huawei-gains-record...

True, local Chinese Android phones are increasing their market share in China, while foreign phones like Samsung and Apple etc. are shrinking.

However, the overall smartphone market is shrinking in China. The economy there is not doing well.