Mr Antonio has some highly developed skills undoubtedly, but I suspect trigonometry is not amongst them. A common oblique holder as supplied presents the pen to the paper at the same angle as a straight assuming the hand position is a constant and the nib is not rotated. It is common for Ornamental Penmanship practitioners to give the flange a bend to reduce the angle of incidence, but it's not compulsory. The only inherent geometrical difference is a marginal one which occurs when the pen is twisted on its axis: the usual clockwise twist applied for OP capital stem shades actually increases the angle somewhat. This is unlikely to be significant. For textbook Spencerian a reduced angle is of very debatable utility since shades are minimal.
Regarding "oblique holders tend to shift the text closer to a 60 degree angle on the right hand of the page", that surely depends on whether you move your paper and keep an eye on your slant? I do not understand that point at all.
The final observation about the fundamental dissimilarity between copperplate and Spencerian is certainly true, but the difference resides in the geometry of the letterforms themselves and the use (or not) of back-and-forth muscular movement, rather than in the writing instrument, surely? After all, both styles comfortably predate the general availability of the oblique pen holder, and both originated as quill-written hands.