According to bitcoinstats.com:
Except for bitseed.xf2.org the seeds aim to return nodes that are currently online and reachable.
If the nodes are not aimed to be currently online and reachable, what qualities (if any) are they aimed to have?
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That DNS server in particular does not exist anymore and is not used by Bitcoin Core.
I believe for a while it returned stale results which is what that comment would have been about.