I'm a Brit since birth so a native English speaker but I heard the expression "dog on the sofa moment" whilst listening to the radio and I have no idea what it means. I could have mis-heard but I don't think so. The context was the presenter saying how she had the opportunity to interview a certain celebrity and it was a real "dog on the sofa moment".
Her dog clearly really enjoys being on the sofa (like most dogs), even if he doesn’t have any right to be there. I don’t think it’s a common idiom, but the meaning seems clear to me. She’s comparing her emotions to her dog's, using both anthropomorphism and metaphor. It gives a sense of her enjoyment at interviewing this person along with her mild "imposter syndrome".
This expression conjures up the image of a dog sitting on a sofa looking not only happy, but very pleased with itself, in a good way. In addition to joy, a dog might also feel the kind of thrill, and maybe even the sense of accomplishment, that comes with access to forbidden territory. The dog in question could be saying, "Wow, I did it!"