As a former Aviva employee who left to take up a new job last year and has seen their dismissal and resignation procedures in action (I suppose it's conceivable they might have changed since I left in September, but I seriously doubt it), I call shenanigans on this.
The company as a whole is so ridiculously risk-averse and keen on trying to present itself well that there is no way on Earth anyone would have been fired by email like that. Every time someone was lef go they were given the news in person by a line manager.
From reading the article, it sounds like the email that went out was actually a standard "Don't forget to return any company property" thing that goes out to someone who already knows that they are leaving. I have a very similar email from when I left.
I used to work there until September last year, and unless something has changed radically in the meantime, they don't actually fire people using a form letter. When I left they sent me a form email confirming my resignation and linking to an intranet checklist, and it sounds like this was the kind of thing that went out. Believe me, their HR department is far too risk-averse to risk looking bad by firing people by email. Everyone I knew who was let go while working there was given the news by a line manager.
Agreed on the high turnover rate though, which I can confirm they have, although that's mainly because most of the jobs there are shit. The place was also full of silly management philosophies and bureaucratic nonsense, and it seemed to favour toadying and sycophancy as ways to get ahead. Also, I had to deal with financial advisers, many of whom are very nasty pieces of work.
Actually, as I understand it, this was just a standard "Don't forget to hand in any company property before you leave" email. I actually worked for Aviva for over a decade (including in their previous incarnation as Norwich Union) until I left to take up a different job last year, and when I left I got exactly the same kind of boilerplate email to confirm my resignation.
From my experience it's not really any worse than any other big company like that, it's just hugely bureaucratic and tiresome to actually ever get anything done there, and they are one of those companies that every few years hire in a new set of consultants who introduce a new cargo cult management philosophy that everyone has shoved down their throats for a couple of years.
The truth of a proposition has nothing to do with its credibility. And vice versa.