I was wondering the same thing, but I guess it will just have to be another plot mystery. I wonder if the author will make a sequel or an extra explain that.
Or it wasn't leaked at all. The author may have been considering making Shingyouji a complete heel, but the store employee specifically mentions that this other beer comes from a small company. Shingyouji's from Phoenix, which is portrayed as being a big name in the industry. Why hand that info over to a small company? Beyond which, we get more after-the-fact bits that make Shingyouji a more likeable character.
It's not at all uncommon for completely unrelated sets of people with no knowledge of one another to come up with the exact same idea at nearly the same time, and it happens in nature as well.
One Problem: they never revealed who leaked the label's design for the beer. Was it Shino? or was it Shingyouji? And how did that happen? Because, other than sleeping with Shingyouji and telling it to him, how else would he have done it? Or did shingyouji perhaps sneak into his office and maybe see the papers or something?? It almost seems to me that Shino-san has a loose mouth, because we don't know who else could have leaked the secret!!
Quite confused!
Buutttt, other than that, that was a really good manga! Loved it!