fwiu protonmail can sign and encrypt a message when you use the protonmail app, but if you want to use a different app (using the protonmail bridge), and set up pgp signing yourself, protonmail does not let you. op wants to use a different app bc the protonmail one is buggy and could also be less secure

The ELI5 is actually within the article. If you want to encrypt your mail so nobody else can read it, you have to use what ProtonMail is using or it breaks.