Gee…
The way I read it is that all of the Cumulative KB updates and Win version updates are just that… Cumulative.
So I would think you have the benefits of KB4505903 if you already have Win 10 (1909) installed.
If someone else thinks otherwise, I’m sure they will chime in here.
I do admit that KB4505903 is listed separately as an installed update in my Update History. But, so are may other later dated “Cumulative KB Updates” for version 1903 and the later update and Cumulative Updates for version 1909.
Regards 