The warning was right there…
But I decided to ignore it and went ahead to install the Qt dependencies on elementary Hera which is based on Ubuntu 18.04.
Tried to run Telephant and of course it had to fail. Tried to backtrack and removed Telephant and the Qt dependencies only to find that it removed something else which I didn’t pay any attention to and my desktop was all wrong. The elementary theme was replaced by default GTK theme. The elementary tweaks helped restore most of the desktop but the login screen won’t budge. You know, the pantheon-greeter. The font was bigger and the Bluetooth device icon and the battery icons for the laptop and the mouse was all old school GTK instead of the shiny elementary ones. Tried to put back those Qt libraries to no avail. Tried rebooting several times after tweaking elementary tweaks to find myself in the same misery. I tried reinstalling elementary-desktop, pantheon-greeter, lightdm but none of them worked. I even thought of reinstalling Hera haha. But then found this nugget of wisdom where it says to purge the elementary-desktop completely before installing it back again.
sudo apt purge elementary-desktop sudo apt install elementary-desktop sudo reboot
Dropped to tty1, issued these commands, and rebooted to the proper login screen. Salvation at last!
All this because I was looking for a gorgeous Mastodon client. I really wish there existed a beautiful Mastodon app written in Vala for elementary. Pinafore will have to do until then.