At the time and in some cases still today, plenty of advocates still make that claim. Many others just assume the truth of it, if only because to question it would cause cognitive dissonance with the prevalence of cloud hosting. (Notwithstanding that some savvier companies use EC2 much like they would a traditional server leasing provider, using instance types that take up the entire machine. Security and convenience is sometimes a trade-off, but some bargains are better than others if you don't succumb to simplistic, categorical claims. Which is what Theo was railing against.)

EDIT: For context, here's the original post https://marc.info/?l=openbsd-misc&m=119318909016582. It's from 2007, when hardware virtualization extensions were new and all VMMs had to emulate network interface cards and similar hardware. These days virtio devices are common place, which helps to substantially reduce footprint. OpenBSD even has its own native VMM, which of course only supports virtio devices.