When factoring DPS you also need to keep in mind the time it takes for Ifrit to walk to each target, his higher risk of getting damaged, and the fact that he does burning strke every 4 seconds, not 3. It's not fair to just factor in contagion losing 1 wind blade-- that seems pretty biased to only include that.
I'm pretty much basing my example on a closed environment, where they would be attacking at the same rate at the same time (of course hands down garuda wins because of how most mechanics work). if anything, I'm biased more towards garuda since I never ever use ifrit, I'm just making up a possible argument where ifrit could potentially do more damage than garuda egi even with contagion.

but the 4 seconds every attack is pretty much what kills it for ifrit.