The certificates are compatible with XP >= SP3.
There’s a second aspect to XP compatibility, and that’s the protocol and cipher suites your web server supports. That’s independent of the certificate (and Let’s Encrypt). If you want to remain compatible with XP, you’ll need to enable some outdated protocols and cipher suites like SSLv3 and DES-CBC.
Enabling these protocols and ciphers is definitely a security trade-off, but it’s still significantly better than no HTTPS at all, so if you absolutely need XP compatibility, it might be acceptable. The Mozilla Wiki link posted by @ahaw021 has some more details on this topic.
The guide you referenced uses cipher suites (in section “Nginx configuration additions — 2 of 3”) that would not be compatible with Windows XP. You can use the suggested settings from Mozilla’s SSL Configuration Generator if you want to change that. (Select “Nginx” and “Old” first - “Old” being the cipher suite selection that supports XP.) The relevant settings you need to change are