Joseph:
The quoted Biur Halacha refers to shkiah without specifying when shkiah is. That’s the contention by the R”T. R”T holds shkiah itself is later, something the CC doesn’t take exception to.
I quote the actual B”H, for the third time:
“…mi’yad achar haschulas ha’shkiyah, haynu mi’shuh sheha’chamah niskaseh me’eynainu, ho issur gemur shelo laasos melacha, vi’chas vi’shaom lihukel ba’zeh, di’ho safek issur skilah…”
“… right after the beginning of shkiyah, which is from the time that the sun is hidden from our eyes, there is an absolute prohibition to not do melacha, and heaven forbid to be lenient in this, for it is a possible prohibition punishable by stoning…”
Did you even read it?