I was that librarian today and it was hard. I have two gifted children and if someone tells me their child is gifted, I believe them.
No. If you were "that librarian" at our library, you'd be talking to everyone, regardless of age, as if they were 4 years old. ;-)
However, today after I had started my 30 minute story time for 3.5-5 year olds a mother walked her 2 year old in, plunked her down and left.
It left me with a room of 3.5-5 year olds who knew the rules and what to expect plus a 2 year old who didn't know the rules, missed her mother, and was distracting to the other children. No doubt this child has a laundry list of wonderful qualities but I had to stop the story time for her and make the other children wait while I found her mother because she just wasn't capable of sitting and listening.
You probably wouldn't have had any issue with it being a 2yo if the parent stayed and supported their child as necessary. In other words, it's not the inappropriate placement, it's lazy parenting that's the real issue.
DW and I see this sort of thing all the time, and it makes our blood boil. Like at the soccer field... kids not dressed appropriately for the weather, not wearing shin guards, not wearing cleats, don't have any water... UUUUGGGH!