Nope doesn't help at all. My main was hacked yesterday and the guy had set jag and everything (was keylogged). I just sent jagex an email regarding it with my acc info and I got my acc back with the recoveries and jag removed and with my new email set.
All you do is post there, and a j-mod will confirm your account was accessed by someone other than the owner, clear that persons information, and let you recover it.
The original account owner can ALWAYS recover their account. With or without JAG doesn't change a single variable in the equation as the poster above me has stated. Say someone isn't the original owner, but has ALL of the original details (including internet provider, IP address, etc) along with having the account in their possession for a substantiated amount of time e.g. 2 years. Under those circumstances they could overcome JAG and recover the account assuming they sold it to someone else. JAG is made to benefit the original account owners in Runescape.
^This, just buy from trusted sellers with 100+ vouches and do your research, the prices may be a little more expensive but the security is worth it. Id recommend buying from Video (1 example)