I CAN take it that that is literally correct, can I? - ie the 30 days starts when you activate it, not when the activation code was e-mailed (ie as soon as you go into the appropriate MySteinberg page)?
So if you download it but don’t activate it for 29 days, you don’t just get a one-day trial period? 
I’ve got a C5 trial I’ve not yet used (have Studio).
Don’t know how that would work with a grace period.
^
Well, has anyone activated the C5 trial some days after downloading (more than 30 days, even?) and still got the full 30-day trial period?
Brains, do you mean you’re wondering whether it might make a difference trying to activate it now that it’s after the start of the grace period (for C6 purchase)? Or are you even wondering whether you can no longer activate it?
What I’d have expected is that it would work exactly like the e-mail says - which would imply you’d still get the 30-day trial period, even if you didn’t activate til more than 30 days after you download it. But, I’d just like to be sure.
Both, but I there would come a time that the database is cleared (finite number range) but I’m no db engineer let alone syncro dev 
I am pretty sure it does work literally. I used it back when it came out, and it was the activation on the USB key that was relevant, not when I received the activation code.
What is not mentioned is that you are limited to 100 launches.