Partly yes, and partly no.
I cannot run sub5's for 5k atm. (injury a few months ago), I have NEVER been able to run sub 5's for a marathon. I hope that one day I will. There are some people stupid enough (Yes, I am including myself) to start an IM knowing that no matter how perfectly they pace the bike, the run won't be under 4 hours. (My best stand alone half mara is 2h05m, best off the bike is 2h08m?... - And without Fitness Buddy, I think my best off the bike would still be ~2h25m)
Some cannot train enough, maybe we are the people who shouldn't race, but then again, I am happy to be a tri-tourist. as much as the sentiment shits me, I am still happy to say I am the reverse pro-cyclist. (When you are oncall and getting paid to carry a laptop, it shouldn't stop you riding, and if you get paid to ride, that makes you pro, right? Just when work rings, you have to stop - thus paid to NOT-train...)
Getting stronger on the bike only helps so much, if there is not enough run-base to be able to be able to do something with good legs when you get off the bike. For those who can ride well enough, but can pace VERY well, more running will help more. (This is somewhat course specific, the harder the bike course, the more bike time will be required, ymmv)
More training in general will help those 98% of the field. more bike or run, both are needed.
(And I do remember a self confessed 'gun runner' swearing at me as I walked past him one IMMelb, as he didn't have the bike training and pushed too hard)