So, if I'm getting this right, a slim cut requires a high armhole, but a high armhole doesn't necessarily create a slim cut. Makes sense.
Well, I think there's really two different things under discussion here: the cut of the actual sleeves of the jacket, and the cut of the body of the jacket. As a matter of sihouette, these ought to be in proportion or else the jacket will look cartoonish. Yet, in principle, tailors can make the sleeves as wide as they want, and the body as baggy as they want. For example, in New York in the 1950s and 60s, during the sack suit's heyday, some custom tailors offered bespoke sack suits that had very small armholes, but fairly baggy bodies. Â The RTW sack suits had large armholes. Conversely, I suppose it would be possible to cut a jacket with very big armholes, but tight shoulders and a fitted skirt. Â It would be hard to make a slim chest, however, but I suppose it could be done. Of course, low armholes give a coat a kind of "droopy" look which is not slimming, no matter how trim the coat may be in other respects.