In case you did not realize this already China sells weapons to whoever wants to buy them.
Saudi Arabia already owned older Chinese liquid fuel rockets (DF-3) which they supposedly got with help from the CIA as intermediaries.
This much is well known and public. They even showed the DF-3 on a parade in Saudi Arabia.
Allegedly the Saudis have nuclear warheads for them. But the warheads are supposedly stored in Pakistan. Saudi Arabia allegedly funded the Pakistanian nuclear program, so a Muslim nation would have nuclear weapons, and this is a kickback they got as a result.
More recently the Saudis supposedly acquired the DF-21 from China.
The Saudis probably realized they needed to upgrade their deterrent given the War on Yemen and recent Iranian military developments.
Hence the solid rocket development.
The Saudis have also vastly increased their military expenditure to the point they outspend countries like Russia thanks to the War on Yemen. They have increasingly as a result sought to manufacture weapons in Saudi Arabia proper, to reduce imports, and this includes deals with Russia to manufacture small arms in Saudi Arabia and probably this deal with China.