That speaks to it being a game shaping, but not being an intentional design decision.

I'm familiar enough with SCBW to have played on ICCUP (albeit as a D player)

No other game in that time (C&C or AoE) had a selection cap.

Starcraft / Starcraft BW is very unusual as far as early RTS systems go, capping at exactly 12 units. When all your peers allow for infinite selection, and then you come out with a design that only allows for 12-unit selections... its clearly an explicit design choice.

From there, we need to reverse-engineer the reasoning behind the design choice.

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I too was a D player on Starcraft:BW. So much training and effort and yet only got to the bottom ranks...