OSRIC relied on both copyright and contract law. Those interested can read the introduction in the freely available PDF.
I mostly agree, but I think the blogger did have a point concerning some expressions of mechanics and copyright. Many of the OSR games, Swords and Wizardry for example, express mechanics that are based on OD&D rather than allowed IP copies of mechanics from the SRD.
My own view is that, in the absence of the OGL, it would be a brave person who published a pocket Players Handbook for 3.0 or 3.5 - it may be possible to express the underlying rules in a way that doesn't infringe on copyrighted text or story elements, nor on trademarks, but I think it would be non-trivial to do so, and not easy to be confident that you had got it right. Whereas the licensing of the SRD under the OGL changes the situation completely. As Ryan Dancey himself conceded, the only thing keeping WotC ahead of rivals in the publication of 3.0 PHBs would be WotC's capacity to produce a more attractive physical product at a cheaper price.
The retro-clones are pretty clear reproductions - content-wise if not layout-wise - of the B/X, OD&D, AD&D etc rulesets. In the absence of the OGL, I think one would have to be similarly cautious about publishing them. For instance, OSRIC v 2 on p 195 has story elements about gnolls and flinds (eg their social structure and political alliances) which are taken straight from the AD&D Monster Manual. And it has stuff about gnomes, on p 5 - for instance, that they can be fighter/illusionists, but if fighter illusionists may not wear any armour better than leather - which combine story elements as well as mechanical elements. In the absence of the SRD being released under the OGL, would this be breaching WotC's copyrights or not? I wouldn't gamble my own bank account on the suggestion that it's not. Whereas the release of all this stuff under the OGL as part of the SRD provides the basis for OSRIC to reproduce the AD&D stuff as open gaming content.
If it was really about copyright law, none of these games would need the SRD and the OGL. Yet they all use them. Of course if in doubt one goes for the belt as well as braces, but I still find this pretty telling.
From WotC's point of view, there may even be an argument that some of these works are in breach of clause 5 of the OGL, because the authors of OSRIC are representing themselves as having authority to introduce, as OGL, content in respect of which they in fact lack that authority (eg the stuff about gnomes). The gnolls stuff is interesting too, because OSRIC declares it as Product Identity to the extent that it is copyrighted work and not derived from the text of the SRD - but to that extent it's arguable that WotC owns the relevant copyrights, and hence that OSRIC is in breach of those copyrights.
Even in the absence of the OGL and SRD it may not be worth WotC's while actually trying to sort all this stuff out. With the OGL and SRD in the mix it's almost certainly not worthwhile - and OSRIC probably helped them sell copies of their deluxe 1st ed AD&d reprints.