The second one, https://github.com/dompdf/dompdf is the original core library, a pure PHP library.
You need the second one, which will automatically include and use the first one, upon proper installation.
The DomPDF core library has more recent commits, but the newest released version (at the time of this posting) is 0.8, which is the same which is used in the wrapper library for Laravel currently. The more recent commits in the DomPDF core GitHub repository have not yet been made to a new release, but it seems from the commit history the features you mentioned have also not been implemented in those newer commits yet.
So the Laravel wrapper DomPDF is using the most current released version of the vanilla PHP library DomPDF, which is fine.
The DomPDF core Readme says:
Handles most CSS 2.1 and a few CSS3 properties, including @import, @media & @page rules
(bold emphasis mine)
So not even CSS 2.1 is completely supported yet. There are missing features. I assume flex and float are among those as of yet unsupported features.
You would have to wait for them to be included in the DomPDF core library in some future version, and then, for the Laravel Wrapper to be updated as well to that new version.