I have a spare room in my house sat idle 80% of the time. I could easily install a few racks of servers in there and have some gigaflops of computing power to contribute.
But actually, being “idle” isn’t the same thing as “spare” because it would cost a lot of money to install racks of servers, just as it costs money to run computations on an otherwise idle computer.
So I mean no, but no. It’s only spare capacity in a stupid, convoluted way that’s disconnected from common sense and common usage of the word.
But you managed to brag about having hundreds of gigaflops of taxpayer-funded computing at your fingertips so good for you, you go for that validation from strangers on the Internet.