Atomic wallet is not completely open sourced - it has several open source libraries[1] but some parts of it are still kept from the public, the fact that not everything is made public still makes it possible for it to have a backdoor.
I didn't say it's open source but noncustodial cryptocurrency wallet just like trust wallet which is trusted and used by millions around the world. If the wallet gives you seeds it doesn't mean it's safe as some of them record those seeds or are not generated randomly enough.