There are a ton of variables involved here, congrats on your setup working well but that doesn't really mean much in the context of other people's issues with high-VG liquid. Who knows what the seller's true motivation was but it's very common for people to avoid high-VG liquids in tanks. It makes much more sense to suggest a
juice that he knows will work compared to a juice that some people might be able to get working but others not. You'd probably be more upset if you were one of the people that can't get high-vg to wick well and then the seller would be a jerk for not suggesting a thinner liquid.
A few of the variables involved which could explain why others have issues and you don't:
"Max vg" juices can vary quite a bit in thickness, one might work fine and another not.
Airflow settings and the strength of your pull determine how much juice is drawn into the coil head. If you have your airflow open more, it draws less juice. If you do a softer pull white taking a drag, it draws less juice.
Not all coil heads are equal, some wick/vape better than others. It's still just people's hands putting these things together and there are going to be better and worse examples even within the exact same manufacturer and model.
Power levels are another thing, you're having success in the mid-30w range, many people vape their tanks in excess of 40w. The more power you are running the faster your wick will have to keep up, and the more issue higher-VG liquid can become.
You could be pulling harder,
vaping at a lower power, and using a thinner "max vg" juice compared to someone else who is having wicking issues, there could be any number of variables which make a direct comparison of wicking abilitiy pointless.
Not to mention the fact that you are rebuilding the OCC heads... many people use the stock heads in their stock configuration since that's the big draw of the new sub-tanks (not having to actually do any work to get there) so you can't really compare wicking capability of a stock head vs a modified one. I can (and did) modify a kayfun so you can do lung pulls at 30w with high vg liquid. I would never, ever suggest someone with a stock kayfun try to do anything close to that, or judge the kayfun's performance based on what a modified one can do.