China saw through the Myanmar generals long ago and know they cannot be trusted, which is why China has effectively broken its own core non-interference policy by arming and supporting the local anti-government militias.
China is content to just let them play their games because at the end of the day, if they step too far over the line, China can and will regime change them. It has an alternative government with its own army ready to go as and when Beijing cares to give the green light, at which point the ranks of their militias can expect to mysteriously swell by orders of magnitudes almost overnight, and they will also suddenly discover all kinds of new advanced weapons systems at the back of the proverbial garage when they were doing their spring clean. If that wasn’t enough, China can easily engineer a pretext to deploy its own formal military under a whole host of pretexts, ranging from self defence when ‘Myanmar’ shells and missiles fall on Chinese territory, as has happened before. Through to humanitarian grounds or even in defence of democracy to undo the coup.
The Myanmar generals knows all this, as well as the fact that the west simply cannot and will not engage in open ground war with China on its own doorstep, which is why they will always need to consider Chinese interests and bottom lines no matter what deals they strike with the west.
China is already serving them notice and warning with its UN moves to not auto-veto any resolutions, so that will come as a massive wake up call for the generals.