The sign must be wrong, or the plant must have passed on - there appears to be what could be a dead looking trunk on the ground right behind the sign.
check this link:
http://www.agaveville.org/view...
A. thraskii, simply is a much bigger plant even at the size of the Aloes in your pictures. I agree with Baja that those plants look a lot like A. cameronii.
The other sign in your picture is an Aloe name (Aloe carronii) that does not return much when I look for it on some of the plant lists out there, could that be a misspelling of A. cameronii?
Botanical gardens do make mistakes.
