The CMA stipulated why they have jurisdiction in their complaint. I don't think anyone contests that.
Microsoft Ltd is a firm in the UK.
If MS breaks UK law by proceeding with a prohibited acquisition, they will be in court, fined, assets frozen, sales frozen, damages liable and on and on.
From the 5th largest economy in the world. Meta bought giphy - the CMA insisted on divestment of giphy and rather than divest, Meta shutdown giphy's advertising arm just to avoid falling foul of UK regulation.
In this impossible hypothetical scenario, MS will just make ABK games mulitplat in the UK, and continue to make money there rather than lose it, and continue on.
Oh and if the CMA is prepared to prohibit the acquisition, the US will follow suit. Regulators don't work in isolation on these global matters.
In reality CMA has to approve or prohibit, and MS have to concede enough to get that approval, or back out.