Logically, the longer chain cannot cause a problem. This is because your 11-28 cassette already includes an 11-23 cassette inside it. The longer chain is working just fine with the 11-23 which are there now. It will work exactly the same if 11-23 is the entire range! If you have no droop with 11-28, there will not be droop with 11-23 with the same chain length.
The other direction doesn't necessarily work, of course. A chain length that is fine on 11-23 might not be long enough to go around the larger cogs of an 11-28, causing a disaster.
Since your derailleur has a spring-loaded tension wheel, extra chain length doesn't matter much. Shifting can be excellent at a range of tensions. So while the 11-23 gives you the opportunity to shorten the chain, you don't have to take up the offer.
If you make the chain as short as possible, then you can't do an emergency repair out on the road which involves splicing out and throwing away a damaged pair of links. (Or rather, you can, but probably shouldn't).