I would not count on that. For starters, I can't find that claim anywhere on their website, and that doesn't surprise me.
I was assuming from their comment...
2. The same buoyant properties as a standard buoyancy aid.
... that it replaced a life jacket. But you're right, it doesn't specifically state it's a life jacket replacement.
I thought immersion suits started at the £400 mark. Thanks for the encouraging news.