lubuntu.net is one of many 'fan' sites which are not related to Ubuntu, or Lubuntu teams. They do provide their own downloads, however most of their downloads do point to official ubuntu servers so it's on you to verify it's an official download (and not something else as it's not an official site).
If you use google.com to look for sites; you'll often find the fan sites listing above official sites, and the site at the top will most likely be one that uses your own language (the official site is really english based, so it's more noticable if you don't use english and a fan site is available in your own tongue).
If you don't know which is the official site, I'd suggest going to ubuntu.com and looking there. You'll end up at https://ubuntu.com/download/flavours which will point you to the official Ubuntu flavor site.
For Lubuntu it's http://lubuntu.me/
I'd trust the site which is controlled by the Lubuntu team, and monitored by Ubuntu, as against alternate sites which are not.
FYI: Lubuntu isn't trying to encourage Lubuntu 19.04 installs at this very late stage of it's life, it's EOL is approaching very fast, so why would you want to install an OS that reaches EOL in a few weeks? (https://lubuntu.me/disco-released/, http://fridge.ubuntu.com/2019/04/19/ubuntu-19-04-disco-dingo-released/ EOL = end 18-Jan-2020)