Doesn't the "childhood friend never wins" meme mostly apply to het rom-coms? The childhood friend is rarely a romantic rival anyway in yuri.
But more importantly, I just don't see the relevance here, as there's more at stake than Hinako's romantic preferences. A "win" for Shiori would mean Hinako's demise, which seems to me like a bigger issue than Miko's "loss" to Shiori.
I'm still rooting for Miko here, but it's not entirely impossible that Shiori just might in the future decide that she has better things to do than eating Hinako.