While this is true of many projects, F-Droid has a track record of sourcing funding for security audits. To date there have been at least three audits, in 2015, 2018, and 2022.
I was involved in addressing in issues identified in the first one in 2015. It was a great experience, much more thorough than the usual "numerous static analysers and a 100 page PDF full of false positives that you often receive.
I'm surprised that several audits didn't uncover this signing issue. GrapheneOS devs do not recommend f-droid. Instead, Play Store is the safest option for now, after Aurora Store