Like many things, perfect is frequently the enemy of good. It sounds to me like the threadlocker itself for a rifle scope and attachments is actually pretty non-critical, ie any half-assed threadlocker-ish stuff that gets sort of dry and kinda holds things together will likely work **well** and provide improvement over no threadlocker…and that the more critical element where failure is most likely to happen for **most** people is in the expiration/fake product/exact application, etc of a product that is less forgiving of variance in those things. If this is even partly true it makes perfect sense to me that something as imprecise—but forgiving—as a paint pen could be **more** reliable under the typically un-exact selection, storage, age, application, etc of many shooters.
Regarding threadlocker not being recommended by manufacturers (someone brought this up a page or three ago) there was a recent thread on the topic—I contacted several scope ring manufacturers, all of whom said that it was fine to use a threadlocker and that the reason they dont recommend it was because customers complain after they get threadlocker on their stuff, ie it was a customer service issue not a functional recommendation in any way.