If it doesn't authenticate anything then it is worthless
Oh but it does. A DV certificate authenticates that a computer responding on behalf of a domain is who it claims to be. Let's Encrypt is perfectly fine for that and does not issue any certificate with information that isn't completely authenticated. This is why they don't issue OV or EV certificates.
The S doesn't indicate anything to do with security and encryption. It indicates a different protocol to standard HTTP is being used. The various results of different certificates are shown differently by browsers. e.g. DV certificate gives you a little green padlock, and an OV certificate gives you your fully qualified organisational name in the title.
You not understanding the difference doesn't make a CA any less trustworthy, and Lets Encrypt demonstrates 100% authenticity of the certificates they issue within the scope of what that certificate is intended to do: Prevent MitM attacks by authenticating the end point as the correct *server*.