Keep his other abilities in mind. For example, a normal W with those two items has a 70% AP ratio. With fury, 85%.
Granted, unless he's ignored he can't really make use of his ult's constant damage, and having to rely entirely on items for one ability to do damage isn't always great.
Also, that's technically not trolling; disagree with me if you will, and yes if you would like you can call me a "white knight" or whatever. Personally, I don't agree with AP Renekton, because the ult's ratio isn't really enough to justify it, but in a normal game, a player has the option to try whatever build they want. The game isn't just about winning with the same time tested builds, but winning with a build that you can make work. Blue Ezreal didn't always exist, for example.
Again, let me be clear: I don't agree with his statement "as long as there is one ability that scales with (you have two "withs") AP, while I may not agree with the strategy, you can build AP." Well, actually, I do agree with it, because it does state that you don't have to agree with the strategy. The point is that players should have the right to experiment and play champions however they like, as long as they have a reason to believe that it can somehow work. I can't see AP Renekton working, simply because to play it you either have to be wildly ahead, or build tanky anyway, making you a worse AD Renekton.
For reference, I can think of a few champions in which building AP isn't the default build, but still can work on them:
. He has a 100% ratio on both his Q and ultimate (activation). If you ever played against him in one for all, you'd understand to respect his range for more than just the rest of his team.
. I believe AP was actually the original build on him; he has quite literally every ability with an AP ratio. He simply lacks the waveclear to compete with mages, for the most part.
. He only has a 100% ratio on his W in mini form, and I believe something like 50% on his ult in mega form. Typically, you build things like a nashor or guinsoo since attack speed will help you proc this bonus more often. It can be quite scary, but does pretty much lock out his mega form, barring the CC.
; this one is a bit arguable considering the amount of scaling that needs to be done. His W has a 90% AP ratio, meaning every time he activates his W he'll pretty much recover a second health bar, and his ultimate starts with a 120% AP ratio but can scale up to 210% with the upgrades, while being completely global. A team that can force enemies to stay in this ultimate will take quite a hilarious amount of damage. I used to play it quite a bit, but because of all the ultimate nerfs (cooldown, mostly), as well as the pacing of the game changing, it's become quite a bit more difficult for me to pull off.
; his blight has a 2% hp per 100 AP ratio, and his ultimate has a 100% AP ratio, that also auto stacks blight on the target over the duration. If they don't build either hilarious amounts of health or MR, they can say goodbye to their health bars.
There are others, of course, but the point stands. Gnar really only has 1 AP ratio, but it works because you aren't required to constantly stand near your target for 10% of your AP as damage (meaning a bonus of 80 every tick if you have 800 AP...)