It’s legit but not complete, it’s not the official form (amtliche Fassung) and only the current laws, not the historic view. In Austria for instance RIS is much more complete: https://www.ris.bka.gv.at/
Edit: not directly related but a fun fact: in Switzerland you can get a copy of the constitutional laws for free at any bookstore and it’s supposed to be understandable for normal citizens and not only for lawyers.
Interesting, did not now about the amtliche Fassung. But the full archive with the amtliche Fassung is also published online for free, starting from 1949: https://www.bgbl.de/xaver/bgbl/start.xav