I definitely don't think that's the case. I'd check out the sections on microG and custom ROMs here: https://madaidans-insecurities.github.io/android.html (they're very knowledgeable, so usually factually accurate, but often frame or evaluate things in a very biased way IMHO, so be sure to evaluate the verifiable facts they state for yourself, and your own threat model). They're way more open to exploitation and far less secure than OEM ROMs, typically because of the way they have to pry open security stuff to get their various hacks to work. They are typically much better on privacy, but having your phone, from which you do all your communication and banking, and which has a dense cluster of sensors and transmitters that follow you wherever you go, be more open to exploitation by any random hacker or piece or malware seems like a bad idea to me.