Brake fluid gets dirty over time (the cr@p that seizes up your calliper pistons works back past the seals etc) as the fluid doesn't circulate the system the dirty fluid 'lives' in the calliper region of the brake line.
Force the piston back without loosening the bleed nipple and you'll push all the crud back into the ABS module.
The module seals are incredibly delicate and not replaceable, so new module time I'm afraid.
I have to admit I thought about this a fair bit since I read it and I would suggest it's FUD spread by dealerships/manufacturers trying to scare people into having an easy home job done by a garage.
Let me explain.
With the bite on spending people are turning to doing these jobs themselves as they are quite easy with the help of a Haynes manual and some basic tools. They are also one of the most lucrative jobs for a garage with Menu pricing. This is where they charge you a fixed price for a service. For instance a brake pad change is based on an hours labour and parts, now you get charged for the hour whether it takes an hour or not. So a competent fitter with a ramp can easily do a set in 30 minutes, thereby upselling his time by double. This means he can do two cars in an hour, thus earning the garage double for absolutely nothing, it's like free money.
Now if they can scare people by saying just pushing back the piston with grips or a clamp (as most fitters and home mechanics currently do) is bad (i'll explain why it's not in a bit) and that you must open the bleed nipple to do it thus opening a sealed system that then MUST be bled (this is no joke if you open a bleed nipple you must bleed it). This would put off probably 50% of people from even trying it and therefore getting them back in the workshops paying £75 per hour and as they need changing every couple of years on every car (and most stuff on the road now has ABS), that's a big chunk of money to miss out on.
Why is it not so bad? It's quite simple really and sound in scientific principle.
This is gravity, viscosity and the speed you can actually force the piston back in.
Gravity because the sediment will fall to the lowest part of the system and the ABS pump is located up in the top of the engine bay with brake lines going up into it for the very reason they do want debris falling down them into it.
Viscosity and the speed because no matter how quickly you push the sediment will still fall towards earth (because of gravity) through the fluid faster than you can push it through the lines using grips or a clamp. As for damaging the ABS pump I don't think a you could push hard or fast enough to damage one with pressure and they have relief valves built in otherwise brakes would bind on solid.
Now add in the internet, the best medium for FUD spreading. I mean all it takes is for one or two of these posts made on the web causing a few to panic because they don't understand the science and design and it spreads all over. Then 'bang!' the garages get the work back because people are worried about messing up a really simple job and causing a massive repair bill.
So far in a internet search I have not seen anything that says it's proven or demonstrated a failure because of this and there is no evidence that it does. The sites I have read say 'could' or 'might', but this is enough to cause FUD.
I have changed pads in at least 6 different cars (equipped with ABS) without opening the bleed nipple and never in 400,000 miles of driving had an ABS fault because of it (most ABS faults are caused by faulty wheel sensors and I've never had an ABS fault at all). Also the thing in my Avatar uses a road motorcycle calliper and each year I replace the brake fluid and they is very little debris in it despite being used in a very harsh environment, why because the seals actually work quite well because they are just that, seals, if they were that poor moisture would get in (it does over time but you water molecules are smaller than dust and it keeps moisture out so that the fluid only needs changing every 3 years or so) and moisture is far, far worse in a hydraulic braking system that a little bit of debris.
Now if you choose to believe it it's up to you but I for one will call BS on it.
Carl.
