I do not know what does mean "fully compatible" in such a context. For
example, ASCII as designed allowed (please note I did not write "was
designed to allow") the use of the 8th bit as parity bit when transmitted as
octet on a telecommunication line; I doubt such use is compatible with
UTF-8.
The parity bit is not data; it's a framing bit used for transport/link purpose only.
ASCII is 7 bit only, so even if a parity bit is added (parity bit can be added as well to 8-bit quantities...), it won't be part of the effective data, because once the transport unit is received and checked, it has to be cleared (so an '@' character will effectively be equal to 64 in ASCII, not to 192 if a even parity bit is added.)
By saying UTF-8 is fully compatible with ASCII, it says that any ASCII-only encoded file needs no reencoding of its bytes to make it UTF-8.
Note that this is only true for the US version of ASCII (well, "ASCII" is normally designating only the last standard US variant of ISO 646, other standard national variants or proprietary variants of ISO 646 should not be named ASCII, but more accurately, for example, "ISO 646-FR:1989", or without the ISO prefix if this is a proprietary charset and not an approved charset published in the ISO 646 standard).
