Remember -- these are the same "SEO experts" that will happily tell you that you need to host every web site on its own IP address, even though a Google employee has said unequivocally (over 10 years ago!) that this has no impact on ranking.
I think you'll find the SEO community to be pretty scientific.
Presumably it's Matt Cutts you're referring too - I'm pretty sure he's lied before about optimisations; Google after all don't want people to know all the details to avoid gaming the algos. If shared IP is a strong signal for low quality sites then it's to Google's benefit to avoid confirming that.
