WordPress is licensed under GPL, which according to WordPress means that all WP plugins must also be licensed to be GPL compatible. There is debate about that, but almost all of the plugins in that person's GitHub account are explicitly licensed under GPL.
Under GPL it is technically legal to redistribute (including selling) the actual code of any plugin - premium or not. There are plenty of sites that do this.
From the GPL FAQs:
If I distribute GPL'd software for a fee, am I required to also make
it available to the public without a charge?
No. However, if someone pays your fee and gets a copy, the GPL gives
them the freedom to release it to the public, with or without a fee.
For example, someone could pay your fee, and then put her copy on a
web site for the general public.
Challenges to this practice usually stem around the use of the plugin author's branding/trademarks/images, which are not covered under GPL. I could technically sell WooCommerce's premium plugins' code, but I would need to remove all references to WooCommerce's trademarks and branding.
So to answer your questions:
It is allowed?
Can I use this Repos for my Composer Setup?
Should I use this?
Morally, this is a gray area. I like quality plugins, so I prefer to ensure plugin authors are getting the compensation they request for the plugins I use.
Why nobody notice that?
People do notice, but it's hard to do anything about it.