Your confidence shows you might have a deeper insight into the rules than I do.
Earlier, I said, "The idea that proficiencies granted by a class feature is not a proficiency "by virtue of being a member of that class" is unsupported in the rules." Your confidence here indicates that you know where to find this distinction in the rules. Could you help me out with a quick page reference so we can clear up this ambiguity?
I'm only about 67% confident on this one, I just haven't seen any new arguments/evidence for a while.
The two statements in the rules that provide the best basis for taking a position are:
1) "When you gain a level in a class other than your first, you gain only some of that class's starting proficiencies, as shown in the Multiclassing Proficiencies table:"
2) "When you gain a new level in a class, you get its features for that level."
Also, the table on PHB p45 is important, because it tells us what the armor and weapon proficiencies are for each class. For cleric, those are light and medium armor, shields, and simple weapons.
It all hinges on whether you consider the (same) list of proficiencies contained in both the table and in the "Proficiencies" section of the class entry to define what a "class's starting proficiencies" are, or whether you consider the "class's starting proficiencies" to refer to any proficiencies a particular character could derive from taking their first level in that class.
Which position is taken will also influence how one reads rule 2 above. From the position that starting proficiencies are defined in those two lists, other features of the class are not starting proficiencies, they are additional class features which are not limited by the multiclassing rules. From the position that they are same thing, that statement is limited by statement 1.
The strength of the defined "class's starting proficiencies" list position is that there are two lists that specifically call out the same list of class proficiencies. Additionally it provides more consistent results between domains--since some domains are significantly better to multiclass into if you can't get the extra proficiencies from the domain. It also maintains a consistent mechanism with the way additional subclass proficiencies can be gained from classes that don't get their subclass at 1st level (it doesn't make sense to me that they intended to nerf the cleric's subclass features simply because they get them at 1st level rather than 2nd or 3rd like most other classes). It also indirectly invokes the rules principle that subclasses always add to, never take away from, your base class, and that they are intended to be something extra and above.*
The strength of the "character could derive it from that class" position appears to me to be that it gets around the weirdness of being able to pick up better armor proficiencies by multiclassing into cleric than into fighter. The position is also tenable because there is no statement in the rules that appears to unambiguously demand either interpretation.
Based on the overall relationship principles between classes and subclasses, I think the defined "class's starting proficiencies" list position make more sense.
*While I haven't seen it brought up anywhere, you could actually play the game without subclasses entirely, and all the rules would still work, as far as I can tell. No class
depends on its subclasses (unless I'm forgetting a class feature somewhere).