It's the other way around - you should be asking if Xray is built for the version of JIRA you are looking for.
You should have found https://marketplace.atlassian.com/plugins/com.xpandit.plugins.xray/server/overview simply by typing Xray into the search though.
Read the marketplace entry, it says "this is not available for Cloud".
It sounds like it is on the way, but it's not available for Cloud, whatever the Xray site says. (Also note the "available soon")
No, it's not weird that a "component is available, but not the component's platform". A lot of add-ons are written for Jira alone, JIRA is their "platform"
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