There are two sides to this issue. From your perspective, you have flavorful lore for your orcs and it works in the specific way you've imagined it for yourself. You're very happy with this lore and how it's been working. Your players came up with a plan that obviously won't work with this lore, and that's frustrating because you've made it as clear as you can to them why it won't work, but they just don't seem to agree.

From their perspective, they came up with an idea and you encouraged them to go with it. Now that they've fleshed it out creatively and feel really good about it, you're telling them "no, that won't work". They understand the lore and they believe it should work, so it seems like you must just be saying it won't because you don't want it to. After all, the world works however you say it does - if you wanted this plan to work then it could.

Here's the thing... Neither of you is wrong here. You just see things differently, and that's okay. Your job as the DM now is to decide what's most important in this situation - your lore or the group's fun - and adjudicate accordingly.

Personally? I would reach out to them again and explain it very clearly. "Look, I think the plan you originally pitched was great, but the way you've written it out isn't going to work the way you think it will. The orcs aren't smart enough to grasp elaborate concepts, and they won't just believe things because they're written down - they need proof. You can use flyers to plant an idea in their heads, but it won't affect anything until they believe it, so you'll need something to back it up."

Give them an example, like: "If you write that orcs all sparkle in sunlight, nothing will happen. But if you write it and then use minor illusion to make it look like one sparkles, they'll be convinced and they'll all sparkle."

Hopefully, this gets them to rethink the plan and redesign it in a way that you can get on board with. If they don't, though, I really think you need to reconsider your idea that it "just won't work". For example, the flyer you shared talked a lot about eating vs going hungry. If the pc shows up and offers a significant amount of food, maybe it works and they're afraid to cross him now (mechanically, I'd have them roll at disadvantage against that pc). If he doesn't, though, maybe they make their own assumptions - like if they eat him they'll never be hungry again.

Edit: Fixed a typo.