It really isn't.
This is one of the things people simply refuse to understand.
If you saw a guy eating a big Mac whilst claiming to be a vegan would you say he is still vegan?
I don't think this is really a suitable example and would also depend on the context. First of all, being a vegan has very very clear rules (or really only one rule), though even within that, there is some range. If I saw a guy eating a big mac on a daily basis? Then no, he's not a vegan and is just openly lying. If I saw him eating a big mac once in an otherwise unblemished years of being a vegan, then yes they're still a vegan. If its someone making the change from being an omnivore and they're gradually cutting meat out with blips here and there, yes they're a vegan because its in the intent and they're trying.
Its also a dangerous road. The reality is different Muslims will have different thresholds for what they consider to be Muslim. I know muslims who drink alcohol, are they Muslims? How about pre-marital sex? How about not praying? How about only fasting some days in Ramadan? I'm not talking lapsed Muslims, Im talking people who would still class themselves Muslims, follow Muslim teachings in the majority of ways and do a mixture of the things above (ie they may drink on occasion but they also pray 5 times a day, fast, are abstinent etc etc).
This topic is always a bit difficult because as someone else said above, you have some who wash their hands of the situation and say well what he's done is not Muslim at all and therefore that's it and you have others who, like that Thierry guy, are interested mostly in inflammatory content and posting in a way that they must surely know is not conducive to any kind of conversation.
Does Islam say any random person can run up to someone who, rightly or wrongly, has been deemed to be an apostate and written offensive things about Islam, and kill them in the street? No it does not. Does Islamic law allow for the death penalty for apostasy in certain conditions? Yes it does.
I've said before that my wife has an Islamic background on one side of her family and I'm obviously very familiar with her extended family back home. We've also been part of various Islamic communities here in the UK, including via a Sunday Arabic school for the kids. Not one person in those groups has ever been violent. However, some of them do sometimes make statements.....that are unsavoury. These are not particularly uncommon. Some may even be called a dog whistle. Are any of these people ever going to act on some of what they've said and do violent acts? No, I don't think so. However, in certain circumstances, these comments could certainly rile others up, yet they very rarely, if ever, get challenged.
Its similar to what Tucker Carlson for example does or on a smaller scale, if someone is at the pub or whatever making comments about how immigrants are doing x or muslims doing y and its affecting our country. They're not necessarily making violent statements themselves and not actually committing violent acts. Yet their speech can incite others if left relatively unchallenged.
Constantly saying that these people aren't actually Muslim or don't represent Islam unfortunately for me doesn't quite cut it. It allows us to wash our hands of the situation and not look at the root causes of why someone attacked Rushdie with the intent to kill, why someone else in Japan killed the Japanese translator, why someone stabbed and almost killed the Italian translator, why someone shot and almost killed the Norwegian translator, why dozens of people were killed in a fire meant to kill the Turkish translator.
Words do have consequences.