He wasn’t officially on the creative team for Once, and he didn’t adapt the book for Sing Street. But his artistic vision was the jumping off point for both shows, and neither show strays especially far from its respective source. So I can see why people might consider him a shared member of these shows’ creative teams, even though he technically isn’t.
For those looking for more word-of-mouth data points:
I saw the show this week and thought it was beautiful and extremely charming. A few things here and there that I didn’t care for, but overall I loved it, and I don’t agree with the OP’s criticisms.