Yes, I agree.
Noumenon is a limit concept, that's why it's unknowable.
If we begin from a perceived thing (a phenomenon), we see first its appearance from some viewpoint. Later we can see it from other viewpoints like overall form, surface materials, function(s), etc. We can also learn about its internal, non-apparent properties by measurement (weight, spatial dimensions, ...), by scientific research, etc. Gradually we can conceive it from multiple viewpoints, but never from all possible, potentially infinite amount of viewpoints. That would be noumenon, but we cannot have criteria to decide that we know all possible viewpoints.
It's like the concept atom as the smallest undividable entity. It's a metaphysical limit concept; how small entity we ever find, we don't have criteria to decide that it cannot be divided further.