Except the very idea that they owe you anything is so absurd that even if they had a contract document stating that they'd do work for you, they
still wouldn't have an obligation to do so because society has decided that contracts without
some consideration from both sides are not valid. Similarly, even if something you buy comes with a piece of paper saying they don't owe you anything if it breaks, the law generally says that's not true.
Because you paid for it.
But they don't say they warrant their work. They have a notice reminding you that you are receiving something for free, and that thing comes with no support, and is not meant to be fit for any particular use you might be thinking of, and that if you want support/help fulfilling some purpose, you can pay someone (maybe even them if you'd like) for that service. Because the way the world works is that as a general principle, other people don't owe you something for nothing. This is not just some legal mumbo jumbo. This is how life works for everyone. It's clear that they're not being malicious (they're not distributing a virus or something), and that's the most you can expect from them.
Computer security is always contextual, but as a general rule, if you're going to be accepting random input from unknown parties, you should have an expert that knows how to do that safely. And as mentioned elsewhere in these comments, such an expert would already be compiling out codecs they don't need and running the processing in a sandboxed environment to mitigate any issues. These days even software written in-house is run in sandboxed environments with minimal permissions when competent professionals are making things. That's just standard practice.
So they should be proud that they support obscure codecs, and by default the onus is on no one to ensure it's free from bugs. If an engineer needs to make a processing pipeline, the onus is always on them to do that correctly. If they want to use a free, unsupported hobby tool as part of their serious engineering project, it's on them to know how to manage any risks involved with that decision. Making good decisions here is literally their job.