There is NO standard for wrench size vs screw size in metric.
I checked, because it kept coming up.
In point of fact, T9 is a real oddball, there's no earthly reason they couldn't have stepped down to the more common T8, or up to the extremely common, and part of every set T10. They are doing that on purpose, to make the device non-user-serviceable.
For that matter, who's to say the machine screws are even metric? I see what looks like a Starbucks coffee, and that's definitely an L14-20 or -30 twist-lock socket in frame. That plus the accent tells me we're probably in the United States, and the screws might be standard.
Standard generally follows SAE guidelines, which do specify head size for bolt size. Show me a 1/4" bolt I'll show you a 7/16" hex. I don't think they delve into Torx, but SAE definitely would not approve of oddball sizes like T9 when a common size like T10 is right next door.