Maybe so, but under the Act, the burden is on the company to prove that the modification directly caused the failure in question.

They can't just deny deny for no reason. But that's the tactic as consumers are largely unaware of their rights under the law, and they typically get away with it until the FTC sends a bunch of nasty letters telling them that they're breaking the law. See "warranty void if removed" stickers.

There's some good discussion of this very issue in this article from a few years back. They touch on the sort of "overclocking" situation that you're talking about.

https://www.vice.com/en/article/yp3nax/jailbreaking-iphone-rooting-android-does-not-void-warranty

In my personal opinion, even a modification that can be used to push the hardware to operate beyond it's design limits doesn't inherently void any part of a warranty if it's not used for that purpose. Let's think through a few examples.

Does obtaining root on a Windows PC void the hardware warranty since it's now possible for you to install a kernel driver that lets you overclock?

Does the presence of MSI Afterburner installed on a machine void hardware warranties? Regardless of whether you use it for overclocking or just for graphing and monitoring?

Does installing Linux on a PC that you own void the hardware warranty?

In my mind, the car example is a bit different. I don't quite know what you mean by "tune", but I'm going to assume it's something like ECU remapping. To me, remapping an ECU seems similar to the act of overclocking, since you're modifying the physical inputs to the engine itself. And I can see how out-of-spec physical inputs could cause irreversible damage to an engine. But that doesn't mean that every conceivable ECU modification would be grounds to deny a claim for engine failure.

(Disclaimer: I have no experience with ECU remapping so I'm making some assumptions)

As another example, let's say your car comes with an ECU that has some security mechanisms to prevent reflashing. Could the manufacturer deny a warranty claim on engine failure just because you circumvented the security mechanism? Even if you never remapped the ECU parameters?

What if your ECU fails, and you decide to install a 3rd party ECU. The new ECU has no security mechanism preventing you from reflashing, but the map that it comes with, while provided by the 3rd party, is functionally equivalent to that in the stock ECU. Can the manufacturer deny a warranty claim on an engine failure, just because without the security mechanism you could have reflashed a new map designed for more performance than stock? Even if you didn't? Could they deny the warranty due to the 3rd party map? Even if physical inputs to the engine stay within safe parameters?

I think you do raise a good point about the logical connection that Samsung could make between the modification (root) and the failure (battery). But the point I'm making is: the legal burden is on them to make that connection, and show how one caused the other.

On the other hand, some of the questions and examples that I raised are admittedly messy, and might not have a clear answer unless tested by a court.

I guess what I'm saying though is: if the cost to seek legal relief here is relatively low (e.g., arbitration or small claims), then if I were OP I would probably go down that route, personally, and make arguments based on Magnuson Moss.

IANAL

Edit: just saw elsewhere that OP lives in Greece. Looks like the legal principles are pretty similar when it comes to warranties in the EU. The burden is still on the manufacturer to show that the modification caused the failure in question.

https://fsfe.org/news/2023/news-20230807-01.en.html

But under the Act, they have to show that what you did caused the malfunction. The burden of proof is on them.

It's why "warranty void if removed" stickers on products are unenforceable in court.