The altitude is exactly what it says it is. It’s the highest altitude to which the airplane was certified during the certification process.
There are different certification requirements for specific operating altitudes and manufacturers will often certify only to one altitude so as to avoid requirements to fly above that altitude
A maximum certificated altitude does not mean the aircraft cannot fo higher, it just means it is a violation of the regulations for it to go higher.
You see the same sort of thing with maximum takeoff weights. Any plane with a MTOW of over 12,500 pounds requires a type rating, so manufacturers will often establish a MTOW of 12,500 pounds to eliminate that requirement even if the plane could easily and safely fly over that TO weight.
I’ve been to 17500’ multiple times traveling VFR eastbound with a naturally aspirated 2018 G6 SR22. This was usually to take advantage of tailwinds. It’s fun doing 160 kt TAS and 230-240 kt over the ground. No problem.
I’m not privy to the company’s rationale. I would guess that it involves 14 CFR 23.2125 which required “The applicant must determine climb performance at each weight, altitude, and ambient temperature within the operating limitations.” So it might be due to the design of their flight test program. They appear to have picked the highest VFR altitude (17,500 ft MSL) as their target. The highest altitude given in the 3400 lb and 3600 lb SR 22 performance tables is 17,500 ft pressure altitude (found in the time, fuel, and distance to climb chart).
I suspect you are correct. The manufacturer can arbitrarily set a limitation on altitude and there is no need to document anything higher than that altitude.
There are some limits prescribed by the FARs and some equipment requirements, but otherwise, I think the operational limits are set by the manufacturer during the certification process. And that becomes a legal limit for operation of the aircraft.