This is something that I saw on a writing group on Facebook today and have seen many times before.
Googling on "the first book you write will be" the top hit I found that was relevant to this blog was in a piece by the bestselling author Amanda Hocking who says "Almost universally speaking, the first book you write will be terrible."
She is certainly not alone in saying so.
I asked 38 authors how many novels they completed before the one that first got published. I've shown the results in diagram form below.
(click figure for detail)
18 of 38 had the first novel they finished published. These 18 included mildly successful authors such as George RR Martin, Rick Riordan, and Joe Abercrombie.
It should be noted that a good fraction (but by no means all) of these authors had also written short stories and part finished novels.
Before his debut (Dying of the Light) GRRM wrote multiple short stories and 30 pages of a novel. Rick Riordan, who like GRRM published multiple novels before breaking big, also wrote a great many short stories. Robin Hobb wrote lots of novellas and "many many" short stories.
There are also some on that "zero" row who wrote no short stories, no part finished novels, and just knocked it out of the park first go.
The thing is that like almost every bit of writing advice or statement about writing ... this truism isn't true. Writers vary hugely in their methods and results. Statements about writing, however definitively someone delivers them, are a bit like the Pirates' Code ... more by way of guidelines.
With all this said, the first book I wrote was rubbish.