This is a good explanation. VW can only void your warranty claim if the problem in question was caused directly by the aftermarket modification. For example: an ECU tune cannot void a warranty repair claim for a faulty power seat motor unless that tune caused the failure.
https://superchips.com/will-tuning-void-my-warranty/
Where to start?
Let's start with your link and what the aftermarket folks love to bandy about to validate their assertions. They are betting that nobody has actually read the MM warranty act.
The purpose of the MM Act is to preclude "tie in sales" where a manufacturer says "in order to keep your warranty, you need to buy all of your maintenance and parts from us".
In layman's terms, the Act prevents a company from refusing warranty service based on the mere presence of a substantially similar replacement part - so you can go into Autozone and say "I need the replacement brakelight bulb for a 2015 Golf and it doesn't matter if it is a Bosch, Osram or Sylvania bulb.
However, any performance part is not (by definition) substantially similar", so they don't fall under the Act.
As a useless factoid, the MM Act came about because a vacuum cleaner manufacturer (I think it was Eureka) refused warranty service merely because the owner used an off brand bag. In short, the manufacturer must show that the aftermarket part caused, or contributed to, the failure - but remember that the presence of a performance part is not covered, so the manufacturer can say "you have a performance XYZ that caused the failure, so we won't cover it under warranty."
Other consumer laws still require a manufacturer to connect a mod to a failure, but it isn't the MM Act.
Now, a note about "voiding the warranty" - a term that we, dealers and service advisors love to throw around.
There are only a few things that will void your warranty. Water damage, branded titles, cars with so many mods that virtually no part of the car is untouched by a mod and racing.
Years ago, Mitsubishi voided the warranty of a bunch of cars they saw at a track meet and BMW voided the warranty on a car that had been heavily modded and the owner tried to reverse the mods before taking the car in - but left such a mess that it would have taken the dealer forever to figure out what was what.
What we really mean is "the dealer is denying warranty service for that failure based on..."
Accordingly, if your car is tagged TD1 (modified ECU) TG1 (modified DSG ECU - I think it is TG1) or TD4 (evidence of a piggyback tune), the you can assume that the manufacturer will deny warranty coverage on most drivetrain issues - unless the issue is known to happen to non-modded cars as well, but you may need to be persuasive.
If you've modded your lighting, then expect issues with any electrical woes and so forth.
That said, If I had a problem with my car, I wouldn't even think about modding it until I had the problem addressed.
Can we tie an oil leak to a modified ECU? Probably, but there is an equal chance that the dealer will recognize it as a known issue. Do you want to roll those dice?
