True, but there is still a chance that some people are going to read the code and alert the rest of the community about the vulnerability, however, if it was closed source, there is no chance of that happening and people's wallets will keep getting drained and they will be waiting for solutions and answers from you the dev, lol.
I get it that open source does not automatically mean safe, but if it is an open source and well reviewed software, it should be better than most closed source options, with the users good opsec of course.
Open source is one of the most important elements for a Bitcoin wallet, but it doesn't guarantee security if no one who understands code has inspected it or if the wallet isn't well-known. The ideal would be to 'trust while being suspicious', the famous mantra: don't trust, verify!
In other words, try open source wallets that are well-known in the community, have good reviews and have been inspected by the community.
I've never downloaded and would never try Atomic Wallet, it reminds me too much of what happened with Jaxx.
The only multi-coin wallet I'm somewhat familiar with is Coinomi, though I hope they open their code one day. That would make it much better.