I took a closer look and this looks like it mostly is a joke, yes. I suspect they just posted it in response to customers asking about high FPS rates so they can jokingly say that nobody else can beat them on FPS

. I considered doing this myself, but I didn't want to risk anybody actually purchasing an "unlimited FPS" server and being dissatisfied by the lower performance it would actually offer.
The $500 price is a bit of a tell, actually -- since the server would top out at using a single core, and no additional bandwidth is needed, it doesn't actually cost much more than one of our "extreme" servers to run (or more than one of their "extreme" servers, either). They likely priced it at $500 with the belief that nobody would actually buy it.
If you want, I can unfetter your server here to show you an "unlimited" FPS rate. However, as you already saw with 25,000 FPS, the fluctuations (they call it "jitter") will be intense. Fluctuations are a huge headache for performance and matter much more than peak FPS -- they're why many "1000fps" servers from competitors of ours are outclassed by even our 250fps and 500fps offerings (our better bandwidth also plays a part, however).