amdad
Short answer: no
Long answer:
DevFlarum is admin accessible, so anyone can go in and change it as far as flarum allows.
Examining the logs (below) reveals that the landing page extension was enabled shortly prior to your post, so my assumption is they enabled it and changed it to some messaged inspired to scare.
It violates one of the few rules of devflarum
However, usage of clearly malicious words [...] is not allowed.
... So I will likely block them.
xxxxxxxxxxx - [10/May/2017:09:54:13 -0700] 200 "POST /login HTTP/1.1" "496" "0.077" "https://devflarum.xyzz.work/d/1-info" "devflarum.xyzz.work" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.96 Safari/537.36"
xxxxxxxxxxx - [10/May/2017:09:56:24 -0700] 204 "POST /api/settings HTTP/1.1" "350" "0.029" "https://devflarum.xyzz.work/admin" "devflarum.xyzz.work" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.96 Safari/537.36"
xxxxxxxxxxx - [10/May/2017:09:57:09 -0700] 200 "POST /api/extensions/johnhearfield-landing-page HTTP/1.1" "342" "0.031" "https://devflarum.xyzz.work/admin" "devflarum.xyzz.work" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.96 Safari/537.36"
xxxxxxxxxxx - [10/May/2017:09:58:05 -0700] 204 "POST /api/settings HTTP/1.1" "350" "0.019" "https://devflarum.xyzz.work/admin" "devflarum.xyzz.work" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.96 Safari/537.36"
As for being hacked, no. It was just a user logging in, enabling a extension and typing words. Not much of a "hack" when there is public admin access ?
Naturally, DevFlarum has already removed any trace of it (since it happened a short period ago)