The claim here is that vegans are not on any grounds to criticize carnists for eating meat because vegans use cell phones, some of which contain, for instance, conflict minerals, or because they don't farm their own food to eliminate all animal deaths.
If you subscribe to this system, you can never encounter an argument that anything you do is immoral because you can always point to the immorality of the person who made the argument. Therefore, nothing can be labeled immoral under this system, because the labeler's ethics must be always called into question. For instance, if I oppose false imprisonment on this site, then does it really make sense to say "Yeah, well you built your site on GitHub, owned by Microsoft, which has its own ethical problems with oppression"? Okay, great, but what does that have to do with what I just said?
You can justify absolutely any action this way unless the person you're talking to literally walks on water. Even then "Oh well you haven't done enough to oppose oppression so why should I listen to you? You haven't personally flown to place X and done thing Y - or you pay taxes to government Z which does some other bad thing.". There's no limit.
Under this system, no one can ever make any ethical argument to you. That doesn't make you right, that just makes you immune to new information.
What does someone else's morality have to do with your own? If someone else steals, does that make stealing ok? No, of course not. Carnism remains objectively unethical, and you've stated an irrelevant fact.
At best, it's an argument that if the other person points out that stealing is unethical, they shouldn't be holier-than-thou about it, but it doesn't actually change the ethics of the situation of what you do.
If we actually evaluate the actual cost impact and view a lack of fair trade as theft, it's something like $30-100 per phone. Annually, on average 25 animals are killed for every person. This means that if you change a phone every two years, worst case you'll be "stealing" $4 for every animal killed. So then which would you consider more morally reprehensible: someone who steals $4 or someone who throws one baby chick into a shredder alive? Is $4 worth someone who holds a dog hostage in their basement and beats it continuously for their own entertainment? And if someone stole $8 as part of a dogfight bet, would you be as upset about the $8 or the two dogs being forced to rip each other apart? There is just no comparison
Veganism is just such an easy change. You still eat all the same things. Most criticisms of this type are essentially that "unless the vegans overhaul their whole life I'm not doing the simplest change ever". "You haven't changed your entire life and quit your day job to become a veganic small-scale farmer, so I won't order the veggie burger instead of the meat burger".
I mean we don't accept this in any other context. "Hey dumping trash in the woods is bad, just take it to a landfill, or cut it up and throw it out in your municipal trash; it's so easy" "Oh well you drive a car, which causes pollution. Unless you literally walk everywhere to collect supplies for that veganic farm you now run I'm not going to stop dumping tires down by the river". It's just absurd.
If you think that moral consistency is required to go vegan, then why not walk and chew bubble gum at the same time? Veganism isn't carte blanche that washes the ethics of anything that doesn't specifically involve animal products. If something else is unethical as well, then go ahead and avoid that too.