I like the idea of trying to get some clarification on that sentence in the manual, and wording it as such. LCD tv's I have seen do not have this disclaimer in their manuals.
My old Samsung LCD had such a disclaimer and I often saw it in modern LCD manuals.
You don´t have an burn-in issue because of OLED. You problem is a IGZO/semiconductor problem (could be something to do with heat - for the next generation LG seems to use something like the IGNIS MaxLife solution to resolve it). Also from other owners I read, they said like you, that they didn´t have burn in issues with videogames from static objects over hours, which is a problem for many Plasma TVs. According to some reviews the LG OLED TV has a very strange "burn-in" effect. In all reviews they said it disapears over time and the used pixels are brighter than the not used pixels like from a letterbox bar. I guess it has something to do with heat from the semiconductor and the voltage behind. More heat means more OLED brightness.
But LG seems to have a solution in the next OLED generation with a new semiconductor alghoritm to avoid such issues.
From the LG SID paper:
The latest technology called "Adaptive SVDD" which helps to reduce power consumption and also to increase the gray scale for OLED displays will be presented. This method is equipped with "external compensation technology" that is suitable for large-sized OLED displays in that it substantially reduces the number of TFTs and capacitors. This external compensation technology requires arithmetic operation, which needs to be driven externally, of Vth and the mobility of TFTs. Since Vth has a tendency to move toward the positive direction and to have a much wider variation by TFT degradation, the input voltage range has to be considered with a marginal Vth which is expected to increase. And this marginal voltage results in an increase in the supply voltage of a source driver IC and to reduce gray scale. This technology details the overhead optimization that corresponds to the Vth margin so that the power consumption is reduced and the image quality is increased. This technology was applied to a 55-in. OLED TV. It decreased the voltage by 25% and the power consumption by 30.6%. Furthermore, it increased the gray scale by 25.8%.
A new way to drive a 55? commercial OLED panel will be discussed. The method compensates for variations in the on-current and threshold voltage by extracting the TFT characteristics of each pixel of the panel. Corrected image-data composed of gain and offset has been generated, improving the panel brightness, uniformity and lifetime.
Vinnies are noticeable during dynamic content, even if only on what he calls "earthy tones", and that's only from one trilogy of films.
On my last panel it was visible all the time from six hours not broken in at all.
The most LCD TVs from an average user have banding, flashlights or DSE much badder than your letterbox issue with the OLED TV and they don´t see it...