Certificates issued by Let's Encrypt are cross-signed by IdenTrust and are trusted by all major browsers[1]. This is just about their own root certificate. Being cross-signed by an existing, trusted CA is a common practice for new CAs, as it would take years for the CA to become usable in practice otherwise.
Despite a fairly large number of users on XP still (2.5% of total users on some sites I manage), I'll give you that it works on non-obsolete OS browsers. However, those are not the only pieces in the world of security.
Java, for example, only started support as recently as 3 weeks ago (2016-07-19)
