Using a model is posing a live person and/or some objects and setting up lights and painting from that. Artists have done this for years. I've done this. Rockwell did this.

Using photo reference is taking photographs (yours or others') and working from those. I've done this. Rockwell did this too!

Tracing is placing your reference image beneath or over your canvas, and tracing the contours you see in it. Tracing paper, chroma-key, projectors, camera lucidia, tracing the image onto acetate and taping that to your Amiga's monitor to trace it again in DPaint, dropping ref into one layer in Photoshop and working over it in another one, these are all methods of tracing. I've done them all! Rockwell probably did this now and then, with the caveat that a pro's tracing is a very different beast from a beginner's - it's easy for a beginner to just trace the contours with no thought as to how they come together into a 3d shape, and get a drawing that feels dead and lifeless and subtly wrong. Saying someone's work looks like a tracing is kind of an insult.

Rotoscoping is explicitly a process of tracing/referencing a sequence of images to produce a sequence of images rather than a single image. It is related! But this article is entirely discussing the way demoscene artists would reproduce a static image, so roto does not apply here. Rockwell painted static images; he never did this.

(It's certainly possible that Rockwell could have taken single frames of film and had them printed for reference, but that's still not roto. Roto's explicitly an animation process that results in a series of drawings based on your film/video ref.)