Contrary to what is said here (and above about past OS releases where the N-1 installer is removed soon after version N launches) Apple now has an article with links to the store and detailed requirements for getting and installing OS X El Capitan that works. (And yes, even after Sierra and High Sierra are out – and apparently now independent from previous "purchases" of that operating system. It is also not appearing in the purchased tab whether or not it was purchased or not.).
Apple says on https://support.apple.com/en-us/HT206886 - How to download OS X El Capitan:
OS X El Capitan remains available for Mac computers that can't upgrade to macOS High Sierra, or that need to upgrade to El Capitan first.
If you still need OS X El Capitan, use this App Store link: Get El Capitan. To download it, your Mac must be using macOS High Sierra or earlier.
The direct link to the App Store is (currently) here.
Current advice here on this site seems to use the help article HT206886 to check for an updated link if the Mac App Store one fails you.
Given the information in the question, the information Apple provides means currently that on Sierra and High Sierra accessing the link given will provide you the opportunity to download the El Capitan installer from the App Store.
The fact that this currently gives the error "not possible because your current system is newer" can only mean three things: 1. that there is a bug in recent versions of the App Store on Sierra or High Sierra, 2. that this policy has changed recently but the KB article doesn't reflect that change 3. that Apple simply lies on the above page which is quoted accurately here.
As memory tempts me to remember that I did download El Capitan from a newer system, it seems options 1+2 are more likely.
However, this does not apply to the conditions listed in the question by OP above.