Telegram/WhatsApp/iMessage/FB Messenger all go pretty far down the 'this is secure messaging' path, with far less justification. (And for a Chinese iPhone iMessage is completely broken.) Far more people use the big platforms. Getting a significant user base for Signal is a big comparative win, even if the handset security it weak.
Should they be less minimalistic on their website? Probably yes. Would anyone read it? Geeks, yes. Maybe people who are worried. But I think it is a small win at a high cost.
Maybe it's possible to write a basic phone opsec guide and just stick it on medium or something. Rely on the magic of Google to help people find it. (Would Baidu index it? I wonder.)