I thought I had quoted your post in my reply... and I do not know if or how I can add it after the fact.
The article used a Mockingbird in the video. They don't always switch to many songs. I've had some get stuck on a certain species and just keep repeating it. My use of "mimic" was for any bird that mimics. I, like you ,have been caught my times by a Blue Jay doing spot on Copper's, Red-shouldered and Red-tailed Hawks... that unless I actually saw the Blue Jay making the call, I would not have believed it at the time. These are not the only species that mimic...
It is also known that some species of warblers do other species calls. And Black-billed and Yellow-billed Cuckoos have been known to do each others calls as well. One theory is species nesting nearby may be unintentionally "teaching" their songs to young of other species nearby.
In regards to Is that a real bird?....
I can say that at our banding station, we sometimes play a passerine song(often a rarer species), owl or other species call...and I have heard numerous accounts of birders saying they just heard such and such species calling and it shows up on their ebird list when they only heard our recording.Honestly I really don't care... just pointing out that it does happen.
I have also been caught a number of times over the years of times hearing a bird calling further up the path to find a from their birder playing whatever species I was hearing from their phone. Again... just pointing out that it does happen.