* echo -e is not portable across shells, and even across versions of the same shell sometimes. The test output should be generated with printf. https://unix.stackexchange.com/q/65803/5132
* The article erroneously says "If you don't see italicised foo above, then you need to add a custom terminfo." where what it should say is "If vim does not emit this control sequence", or it should have an intermediate example using tput in place of hardwired control sequences. A custom terminfo will not affect the echo/printf command of the raw control sequences at all.
* The sideswipe about "better terminal emulators" mentioning only 3 fashionable ones is not well-founded. In fact, most terminal emulators (from XTerm to Konsole, including even recent VTE-based ones) support italicization. Supporting the boldface/roman/italics triad that has been in Unix document processing for approaching half a century is not a niche or novel thing.
* The xterm-256color entry in terminfo already has ritm= and sitm= capabilities (via xterm-base and ecma+italics). There's no need to make one's own from someone's file on GitHub. https://invisible-island.net/ncurses/terminfo.ti.html#tic-xt...
* The correct terminal type for tmux in terminfo is tmux or tmux-256color, which again already have ritm and sitm (again via ecma+italics). There is no need for a "new entry called 'tmux'". One should already have been there for roughly 5 years now. https://invisible-island.net/ncurses/terminfo.ti.html#tic-tm...