Was that true in 2012?

Very yes.

Depends, it was easily obtainable via OS package managers, and installers in 2016.

However that has been true since 2016, and here we are, 10 years later.

For Linux, but not for Windows and Mac

I mentioned "and installers" for a reason, I know Qt since KDE exists.

Neither existed in 2012, so of course not. On the other hand, QT has always been really easy to download from QT. I don't remember who owned QT in 2012. They've changed hands a couple times over the years, but in 2012 I was downloading QT from whoever on them. QT was large, so you needed a decent sized computer to build it, but in 2012 this was not a big deal.

People get too hung up on package managers. When you're building your own proprietary you don't really need a package manager, you just need a central place to download the tarballs from. Or you could do a monorepo and just build everything locally - in 2012, that means you start your build before you leave Friday afternoon, and when you come back Monday, everything's built.