I'm not a fan of diffusion treatment, I don't agree with it at because it's artificially adding a colour that isn't in the stone to begin with, however, I can confirm that it is not a surface coating.
Diffusion is a permanent treatment. Basically, the stone is heated to the point until its soft enough to add elements that give the stone an artificial colour - so, for example, Beryllium is added to Sapphire.
In the case of Andesine, the stone starts of as Yellow Labradorite, which then has its colour changed by the Diffusion method. Hence why many retailers refer to Andesine as Andesine-Labradorite, because Andesine is a very controversial stone in the trade.
So whilst it is a permanent treatment, it's adding something to the stone that wasn't originally there.
The downside to Diffusion treatment is that you can often notice where the added element hasn't applied the colour consistently. I've seen 'Andesine' that has lots of colour in 3/4 of the stone, but towards the very bottom, the colour is very patchy with, in a lot of cases, the original colour showing.
So in short, the treatment is as durable as the original stone - the only way of scratching a diffused sapphire is the same as an untreated / heated sapphire - with something 9 or above on the mohs scale.
The treatment that is surface treated is the treatment that shows as 'C' on air. That has a coating on the underside of the stone which gives the stone its colour. That's what makes things like 'Mystic' Topaz possible. This treatment can be very easy to scratch.
