It has been many years, perhaps the translator has already changed their mind about their assumption, but it bothers me to see this go unaddressed.
It isn't implied that it was someone else's rule
It is. That phrasing is used to refer to external restrictions, not personal preference.
1, i'm not aware of Sugano Manami having had a male main character in the last 10 years
There are one-shots in the same book.
2, the book is a collection of one-shots and doujins, so it seems unlikely there would be any external requirements
Authors are frequently given parameters for one-shots.