That's because "disable" is the default value. If you do "get | grep sip" under config sys setting, you would see it's disabled. But that doesn't seem to completely disable the SIP session helper. You need to remove the actual SIP helper under "config sys session-helper". I think it's "edit 12" or "13". You should check before removing it. This require a system reboot to take effect.
By the way "set sip-helper enable/disable" was even removed from 6.2. I think that's because removal of the SIP session-helper would do and still be required.
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