Veils (and potentially mental effects) are the only functions of evocation I can think of that are actually called out as being specific to any particular element. Aside from those special cases (which are called out as such in the rules), elements differ from one another almost exclusively in their 'flavourings', ie. themes.
ie. they are thematic groupings.
I see no other way to interpret the statement, "the source gets “tacked on” to his existing spellcasting abilities as an extra area of focus" (YS287; in fact, that whole paragraph leads me to this conclusion, but this statement is the most telling for me), in a way that retains meaning within the structure of the rules, than to interpret it as liscence to purchase Refinement for the purposes of Specialisation in the [thematic grouping] of Sponsored Magic as applied to either Evocation or Thaumaturgy.
If you can provide otherwise, I'd be open to hearing you out. As I admitted earlier, I may in fact be misinterpreting either you or these rules. I do not believe that I am, obviously, but I do admit the possibility, and would be open to substantive rebuttal.