All below is out of my faulty memory. Please correct me if I'm wrong. Looking forward to learning, and re-remembering.
One thing to keep in mind is, that the term Copperplate comes initially from the world of engraving. A metal cutting blade was used to incise grooves into a copper plate, which was then inked and put under a high-pressure press (not the same press as would be used with lead or lead-alloy movable type). The technology involved in metal engraving (the
intaglio process, Italian for
dug in) had existed as far back as the early 17th century (early 1600s) I think. It was useful for illustrations which involved finer lines than could be efficiently produced on a wood carving or end-block engraving, and therefore was often used on title pages and the like for a frontispiece. But it was mildly more expensive, since the page had to go through a different press than whichever press had printed most of the pages of type, and therefore required that the printer take a second impression, costing more time and requiring greater investment in industrial infrastructure. Higher end.
Consequently, just in subjective terms, a Copperplate script in the more general, non-fountain-pen-world sense, is anything which mimics the script found on such engravings. It would have mild variation between broad and narrow strokes; a capacity for EXTREMELY narrow strokes at the skinny end of the range; and an interest in classy legibility above all else. What the fountain-pen (and dip-pen) people have created out of the general-world definition of Copperplate is a specific type face (font, really, is the proper word). Viz. the fact that your computer probably also has a Copperplate font for use that neither engraves a line in metal nor dips a pen in ink.
By comparison, a Spenserian idea came into existence only under the influence of metal-nibbed dip pens. The existence of the pen, itself, allowed for the development of the stylized script. Therefore, it brings to greatest fruition all possibilities of a flexible nib. It's all about pushing the pen to its limits, while nevertheless remaining tasteful (tasteful?
de gustibus non est disputandum) and also quickly performed / produced by a skilled practicioner. The variation between narrow and wide strokes is more free to go crazy; and the presentation is all about flourish within the definition as delineated by pen practicioners.
Those very specific people, living in a very specific era, who thunk up Spenserian on their own, therefore get to define what a Spenserian is or isn't; whereas the development of Copperplate took place over time under a variety of early industrial circumstances, and so will encompass a broader range of styles but also a more narrow range of functions. I might go so far as to say that Copperplate is a precursor to Spenserian, or that Spenserian is a very distant take-off from Copperplate.
Just my thoughts. If you Google "Elizabethan Secretary Hand" you'll get something altogether more bizarre and inexplicable ...
