What matters for Saudis isn't just mil tech, but actual physical presence and protection - and not just against external opponents. The
US is a security guarantor for much of the middle east.
Until China intervenes in the Middle East the way US and Soviet Union did in the mid-late 1950s(basically throwing UK and France out) - it's a hopeless affair.
Saudis might want the most advanced equipment but what they want and what they'll get is 2 different things. China will be using most of those 5th gens themselves, and the will rely on the new technologies to achieve front breakthroughs in a major upcoming war. If the US is not housekeeping it's best tech well, that's their unforced error.
"security guarantor" is whoever is willing to sell Saudis as much shit as they want. Sure that can be China, but if China wants it is another matter.
I don't think China will supply Saudi with arms that are more needed for the home front. At most, they'll keep supplying curiosities that have high value to the Saudis but aren't breakthrough weapons in Asia. Lasers, advanced drones in limited numbers etc.