24 hours transit russia with separate tickets

Yes, as long as the total time in China is no more than 24 hours, which means the scheduled landing to scheduled departure is less than 24 hours, then you are eligible for the 24hr visa free transit.

Just a couple of additional details.

Upon your landing at Daxing airport the first thing that you will do is to approach 24/144 hour visa free transit counter where you will apply for a temporary entry permit in order to be allowed to cross the immigration and continue your journey on a domestic flight. Here you will present your flight itinerary (having your flight tickets as printouts helps a lot) and after a short check, you will be granted a temporary entry permit to cross the immigration, to collect your luggage and to perform a new check-in for the continuation of your journey. This application is free of any charge.

When you arrive in Xi'an Xianyang you will just cross the exit immigration and head to the gate for your flight to Phuket. To conclude, you will be covered by 24 hour visa free transit and you have right to travel on separate tickets and on unrelated airlines. If you find that a person at check-in in Novosibirsk is not exactly familiar with this rule, you will kindly point her/him that this is all presented in Timatic and you can also kindly insist to talk to the supervisor. Have a nice journey.