I guess I'm also surprised that Solus doesn't have xterm, since I've grown used to it being on every other GNU/Linux system I've been on over the years, but note that Solus has a particular use model in mind:
Solus is an operating system that is designed for home computing.
i.e. not servers, headless setups, net installs, etc. Given that every image (Budgie, MATE, KDE) has a default terminal emulator, they expect you to use it.
That being said, there is rxvt-unicode in the repository, which is a lightweight terminal similar to xterm (but better IMHO). Once installed, you can open it with urxvt. (See man urxvt for details.)
And to answer this specific question:
is there no general command for x-believing terminal, no matter which windows-manager is installed ?!
There is: use x-terminal-emulator, which will open the default terminal application. (Either gnome-terminal, mate-terminal, or konsole on Solus). You can use this across most (if not all) GNU/Linux systems to just "give me a terminal!"
On my system:
$ readlink -f /usr/bin/x-terminal-emulator
/usr/bin/gnome-terminal