It wouldn't be a surprise that Apple prototyped something like this. But, the bigger HomePod would have been a bit too heavy with a battery, and a battery in either of them would probably only last a few hours at most. It's not a great user experience to have your speaker die after a few hours, unless you're just sitting on the patio for an hour or two relaxing...and if you do that you can put a speaker out there all the time.
Apple really doesn't seem like it knows what to do with its home entertainment setup. Or, maybe they do, they are just waiting for things to get to a certain point where they finally follow through on whatever their plans are. They still make good stuff, I'm wishing I had picked up a HomePod or two when they went on fire sale for the better sound than the Mini.