Ukrainian officials say their military is now using a long-range missile that was designed and manufactured domestically and can reach targets inside Russia
Or, they've found a viable way to disguise donated long-range missiles so they can be used to strike back at Russia without implicating the country that provided them.
At this point, if Russia stated they were hit with a long range missile from "Country X" and provided pictures of identifiable debris, would anyone actually believe them?
Russia would VERY MUCH care, and so would any NATO nation for what it'd mean. If a NATO weapon hits Russia, Russia will at minimum start actually threatening nuclear strikes, and probably throw down a lot more of their military to start threatening NATO directly.
No, they wouldn't win sh*t anyways, even if they started using the four nukes that are still operational, but a properly thrashing Russia would be very ugly for the whole region regardless of how fast NATO 'could' end Russia.
Sure. Absence of proof isn't proof of absence, but nor is it a proof of occurence. Best we can do is to wait for someone to do an error in order to be able to know a bit more.