From what I can gather from your timeline, you contacted Saga prior to the stated renewal date to reduce the cover level , which prompted the "You're covered" email. I strongly suspect, having had insurance and observed the discrepancies between what they say and the facts* the "You're covered"
email is almost certainly system generated when a new cover commences - in this case your reduced cover, presumably for the remainder of the policy. I do not read it as indicating that the renewal, due to occur about 3 weeks later, had already been actioned.
I would say that you are generally entitled to take what they say at face value, rather that applying a cynical translation to mean something entirely different than what it says, but I, personally, would be minded to treat that as a side issue and not conflate it with the auto-renewal term, which on the face of it appears to extend the duration of the insurance contract indefinitely in 12 month chunks, subject to cancellation with notice from either party.
*A favourite being when, after cancelling the autorenewal that I never asked for of agreed to because I had found and accepted a cheaper quote, the policy expired, receiving a warning email to inform me that I was uninsured - which was a big fat lie, I was insured, just not with them.
