The way you've described it is basically how it would have to work.

Various ad-blocking detection technologies basically boil down to loading some element on the page and then querying for it during/after rendering to see if it's still there. This could be combined with an AJAX call to load the actual content, which is how all those annoying sites work that pop a nag up in your face if you're running uBlock or whatever. And even then you don't get the content even if you subsequently block the nag notice.

A truly undetectable adblocker would still have to pull down and load all the ad content and render it somewhere (invisibly in the background, presumably) and then serve a second cut down version of that page with the ad elements not rendered.

Edit to add: This would be somewhat detrimental to the user, because it would by necessity not stop the types of tracking that are typically built into served ads. Current adblockers (like uBlock Origin) also by default also block various advertisers' nonsense like cross-site tracking cookies and tracking pixels, etc.