Strictly speaking it means the laws against gay marriage were themselves illegal. (As with any law deemed unconstitutional.) It's a matter of word choice to say "gay marriage was always legal" versus "anti-gay marriage laws were always illegal". There's some semantic difference between the two, but I'm not sure what the difference is from a judicial standpoint.

Basically. That's how most constitutional arguments work.

That's basically correct. It's always been legal, and efforts to stop gay marriage have basically been efforts to narrow the scope of this already legal right.