There have been rectangular tabular icebergs in the past so this is not uncommon at all.
Kelly Brunt, an ice scientist with NASA and the University of Maryland said:
Tabular icebergs form, she said, through a process that's a bit like a fingernail growing too long and cracking off at the end. They're often rectangular and geometric as a result
What makes this one a bit unusual is that it looks almost like a square,"
So I think we can categorically state that the coordinates in the plane of the sea are not 1:4 or 1:9 or even 4:9
And we also know that underwater there will be a good 90% of its mass so it's going to be closer to a cube than the famous monolith from 2001.
A better photo, showing the whole berg shows just how non-rectangular it actually is at :
A google search for rectangular tabular icebergs returns a few beautiful examples over the last few years. There is a nice photo by Carole-Anne Fooks here, and I really like this one from icestories.exploratorium.edu: