Can I already install TeX Live 2019 using install-tl?
Debian does not ship the binaries that are shipped in TUG TeX Live 2018, but the binaries in Debian are built from a snapshot of the development sources to get lots of bugfixes and some improvements. I have updated the binaries as far as possible (skipping the incompatible luatex update). Thus, the binaries in Debian correctly refer to themselves as built from the 2019/dev development branch, but are not those that will go into TeX Live 2019 - for this the /dev part would disappear.
TeX Live 2019 is not yet released, in fact it hasn't even entered the pretest phase yet. What you are observing is a “bug” in Debian, that the binaries report the wrong version string1. The version shipped with Debian is actually the same as the one in TeX Live 2018. You can check using
$ luatex
This is LuaTeX, Version 1.07.0 (TeX Live 2019/dev/Debian)
restricted system commands enabled.
**\relax
*\directlua{print(status.development_id)}
6686
And here for comparison from vanilla TeX Live 2018, installed from https://tug.org/texlive/
$ luatex
This is LuaTeX, Version 1.07.0 (TeX Live 2018)
restricted system commands enabled.
**\relax
*\directlua{print(status.development_id)}
6686
1 It's not really a bug because it does not affect the functionality in any way. Calling it a “glitch” or “inaccuracy” might be more appropriate. Also keep in mind that you are using Debian testing.