There's actually no requirement for car seat fabrics to be flame retardant, at least not in the car seat regulations. Flame retardants are basically only useful in the context of something like a dropped cigarette or an overheated electric cable, they don't have any use in a catastrophic scenario like a vehicle fire.
The UK on the other hand requires any kind of "furniture" to be treated with flame retardants in order to be compliant with a fire safety regulation. Because infant car seats are commonly compatible with a pushchair they are classed as "prams" which is a type of bed and therefore comes under that regulation. This causes an issue with looking at ADAC results for this because companies trading in the EU will have a slightly different model for the EU and the UK market and ADAC are based in Germany therefore they test the version of the seat available on the German market, which is different to the UK one.
This only applies to the infant stage. Once you get to car seats which stay in the car they are no longer "furniture" and the EU/UK versions are the same.
It's also why Which? don't use the "Schadstoffe" score when they buy the ADAC scores to give their ratings - they only look at the crash testing and ease of use etc. (Stiftung Warentest do use this because they are based in Germany).
TBH a lot of people also find the ADAC test results in this specific area to be very dubious and/or irrelevant which is why they only look at the crash testing score and ignore everything else. Some brands who have received poor ratings for this in the past have put out a statement about it, you might find this minimising or interesting. The way that they test it is to take some of the fibres from the cover and put them into a chemical solution and then they test that solution - it's not really clear whether that is something that would equate to babies being at risk from these chemicals. But Germans in general are very keen on being "natural" and tend to worry about this kind of thing, more so than a lot of EU countries. I live in Germany and was amused to see that the baby monitor we bought has a section about EMF safety only in the German language section of the instructions - apparently other nations don't consider this a frequent enough worry to include it.
If you do have concerns about these things - Nuna do have an infant seat where the cover is made of wool and naturally flame retardant therefore it doesn't have the chemicals in the cover. They are compatible with most of the common pram adapters.
There used to be a poster on MN who posted quite regularly about the flame retardant law in the UK and how it gets applied to prams and car seats, their posting name was "FlameIngSofa" with that spelling, so if you google or advanced search it will probably bring up their threads, though I expect any car seat recommendations will now be out of date.