Is it that they want Cass to be Batgirl for 5 years - G3 years 7-11 - and then Barbara? Cass's solo title alone did run 6 years, until 2006, and the character continued to exist through 2009. (Or did she? She was with the League of Assassins in One Year Later - when was she reinstated as Batgirl?)
I don't know when she first appeared. But is it safe to say Cass was Batgirl for at least a decade? And now the Cass/Batgirl DC Ink book is about to appear. So, Cass as Batgirl is much more prominent in DC publishing history, though what was the elapsed time assumed to be? (A story can take place over a few days or a week but take a year to be published.)
In this chart, Cass can only be Batgirl for 5 years, and they can't chop much of that out for Steph without "elevating" Steph to Cass's level. I guess they could split it up as 4 years and 1 year. Or 4 years 11 months, relegating Steph to a very short storyline, more like her actual time spent as Robin - brief. Something like "Cass doesn't feel well, so Steph takes over for her for a few days." That's not nothing! To be part of the timeline even like that would mean it "happened." Maybe a day or two, and most of it spent under the influence of Black Mercy. I mean, gooperangs obviously can't really exist. Has to have been a dream.
But so much is missing - are they suggesting Steph was Robin for 5 years, years 9-13, until Damian debuts in year 14? That seems unlikely.
I think there is a lot that is missing and a lot that will change. All of it can change until the very specific storylines matter - unless there are stories directly involving a Batgirl in the era just before Barbara returns, this can be left open indefinitely.