...is it because the hosts are running ESXi 6.0? Do they need to be upgraded to 6.5 to support vSAN 6.5?
Yes, that's why. The hosts and vCenter versions must agree to run a healthy vSAN cluster.
You have answered your question yourself in your post.
This is expected behavior as your ESXi hosts are on 6.0 version and vCenter on 6.5 version.
vCenter is just managing the vSAN ESXi hosts. The Health Service is actually installed on individual hosts. In your case, the hosts have 6.0 Health Service but vCenter has latest health info i.e. 6.5. vCenter can only show warning in this case as It cannot do anything till the time hosts are not on the 6.5 version. So you need to upgrade the ESXi hosts as well to 6.5 version in order to use latest Health Service.
Hope this answers your query!
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