Tell me you didn’t click the link without telling me you didn’t click the link.
Darwinia still sold copies through their site.
Straight from the linked forum post:
As part of the launch and Steam’s exclusivity, we will no longer be offering Darwinia as a download option from our site, although it will still be possible to purchase shipped boxed copies. At Valve’s request we will also be removing the demo from our site for about a month.
So, yes, they were still selling boxed copies - because it was 2005 - but Valve made them stop selling digital copies from their own site and even made them take down their own demo.
It wasn’t exclusive.
Again, same quote as above:
As part of the launch and Steam’s exclusivity
Not sure how you’re getting “it wasn’t exclusive” from a post that explicitly says that they signed their game up for Steam exclusivity.