It wouldn’t be very secret if it was published on the internet. It’s definitely a credible concern given the level of control China demands of companies operating in the country. The US also essentially has backdoors into most communication, and possibly phones as well, so it’s not that crazy for China to also have them.
China is also very aggressive in hacking into companies. Even if they didn’t have a custom backdoor, them finding a way in and essentially banning Huawei from fixing it, is another option.