Not that far, because we are not using 4.13 as it was for the initial release:

there's multiple backport from upstream since then (so it's more recent than that in terms of code base) there's also XenServer/XCP patches on top of it

But yes, it's not using the latest Xen upstream version, for various reasons (also as 8.2 is an LTS, we try to be conservative in terms of new versions/features, stability is more important than latest Xen features).

Finally, Xen ABI isn't stable (yet), so having a more recent Xen version also means breaking stuff with other components inside XCP-ng (XAPI/xenopsd)