I think the reason people might've thought it was promotion was because of how you phrased it: 'Want to build this MOC? Link to my let's build video is here!'. The whole 'let's build' title implied that this was some kind of on-going video series, even if it wasn't.
They are technically allowed, but, there is a fine line between promotion and creative content. So, you'd be better off just posting the images that you used to make the slide show in your moc topic.