I think she's referring to any item that only one is in a home.
When a lot is at object cap, and you remove nonrepeated item(s) you eventually drop below object cap. Then you can place an item not already in home.
If you are building on a lot that has just reached cap, if you want to do more, it becomes a matter of trading off. For example, replacing a unique sofa or bed for one that's already used in the home will for the cap. Then you can add one more item.
It's more problematic if EA suddenly drops the limit of items allowed. You really don't know how many unique items you would have to eliminate to get below cap. If it's a lot, and you just wanted to do a small change, you might want to just stay with what you have, as once you removed things, you wouldn't be able to put them back.