That's not how it works, so I believe hadoop-auth may be working only because something else is quietly doing the relogin...
The renewTGT option is only applicable when using a ticket cache. It will fail if the ticket cache option is not enabled. The option causes a TGT obtained from the ticket cache during login to be renewed before its stuffed into the Subject. Afterwards, there is no automatic background renewal triggered by this option. You have to relogin via a LoginContext to allow the kerberos login module to do the renewal.
The UGI has relogin logic for both ticket cache and keytab. Relogin from the ticket cache triggers the renewTGT upon re-login. Relogin from the keytab gets a new TGT. The latter is critical for daemons. RPC automatically issues a relogin for connection errors, so webhdfs just like hftp, must do the relogin themselves.
I haven't changed the behavior of webhdfs, but rather moved relogin to a common place. The goal here is minimal change to make webhdfs usable beyond 10h. The proposed changes appear predicated on a misunderstanding, so are you ok with this patch?
(Aside: I already plan to streamline all the relogin methods into a single relogin as part of my stalled, but soon to be resumed, SASL work)