Disease X isn't a disease, it's a variable. It's sort of like calling an unknown person a Jane Doe. It's a fill-in term for something unknown. A hypothetical.
Here's a quote from the National Institute of Health's Library of Medicine:
"Since 2015, the WHO has used this designation for a disease that could cause a pandemic due to a pathogen currently unknown to cause human illness. Last year’s Disease X now has a name, COVID-19. The next unknown and unnamed entity may already be lurking."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7390567/
The WHO (World Health Organization) is what is using this term to describe the next potential pandemic cause.
Technically, the statement is true, as whatever Disease X turns out to be could very well cause another pandemic, but the way this is phrased makes it sound like some disease named by Elon Musk is already up and running. Therefore, I'd argue it's misleading.