I'd say that statement was correct some 3 years ago, when plenty of old games started to be sold through Ziggurat, Mirrorsoft, Pixelgames UK and another publisher or two.
Unfortunately it seems that for the last couple of years the "new" old games appearing on steam has been dwindling quite a bit. It could be related to the difficulty in getting the licensing deals or (and that's my theory) the profit margins for selling on steam are extremely low.

GOG is what it is but I don't think steam is the best storefront for selling 30+ years games. Not when it receives an average of 10 (or more) new indie or AAA games every single day (including weekends).

It's been this way for a long time, I remember some frustration about GOG losing exclusives when Square Enix started releasing GOG versions of games on Steam back around 2013. That's why GOG couldn't stick only with good old games.

Oldies have been always better presented, had better visibility and were better polished in GOG.
Outdated and badly configured DOSBOX versions have been something common in Steam. In that regard GOG is not perfect, indeed, but much better compared with the Massive and unmanageable Zoco/supermarket of chaos Steam became.