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.
You are right about the protection, but that hardly means it's hopeless for Beijing. Providing another source of equipment gives the Saudis leverage, which they are of course happy to use against Washington to get a better deal. And even more US resources going to the Middle East instead of the Pacific is a great deal for China.