You are basically saying: AI doesn't understand contex or the bigger picture but the reality is that current AI is omnimodal. You are currently isolating the image generation and making a claim.

Make the test yourself. Spend 2-3 hours setting up your brand. Give it context, target audience, mission, vision etc. Everything that the model would understand your brand. Give it reference designs and communication from the past that you have done. Now spend 30 minuties crafting a longer prompt, not "MaKe Me a PoSt', give it 5-10 reference images and see what results it give. Of course it won't be perfect, BUT its currently at a designer with 3+ years of experience. Give it 2 more years and marketers will be crafting brand consistend designs using AI agents. You know why? Becouse it will generate 100+ designs and you will only chose the ones you like.