As a TD of multiple tournaments, including A-tier. I might have ruled it safe because the rule book and map I distributed had the OB overlooked. A deciding factor would be how many people had an OB there.
If I did rule it safe, I would then make an announcement before handing out cards for the next round (assuming there is one), that the flower bed is OB. Did this TD make the right call, absolutely. It was his tournament and he felt it was clear enough that the flowerbed was OB. I mean really, the previous hole it was OB and it was properly marked on the course itself as a player I would make a mental note, there is OB behind the mando that I could possibly land in.
The caddy book is god for some, I know when I get one as a player, I walk the course and make notes in the book etc...
On a side note, would the player have thrown a different shot had he known the flowerbed was OB?