Using Corbera in an example is difficult. You always have to check the subtlties - pockets, lapels, colors, etc. Earth tones as I understand it have a long history in english country wear with a more casual connotation. The three patch pocket lowers the formality level further, which works hand in hand with the notch lapels to create a distinctly informal feeling. While a flannel or tweed in such a color would probably certainly still be less formal, even in the lightest that probably wouldn't work well for the season (which looks like s/s given the flowers in the background). In such a case, a fresco is a nice open weave that can be light enough to serve the purpose. Given the details of the shirt, jacket, and shoes the pants seem to be appropriately formal when taken in the context of everything else. Overall though, my guess was is that he didn't care, wore it because he wanted to, and will smirk that it's being discussed.
I see that it's a bad choice for an example, as his stylistic choices were just on point for the intended formality. However, I wasn't trying to make a point about Vox's outfit, and if he cared or not doesn't matters for the topic being discussed, which is the cloth's formality.
So, if I got you opinion straight, fresco can be used for a casual coat, depending on the color and depending on how the coat is designed. Agreed, but how about those more suit-ish colors and designs? For example, navy blue sb with flapped pockets and notch lapels.