Seems to me that there is "Brewer's Yeast", "Autolyzed yeast extract", and "hydrolyzed yeast extract" which are all dead, i.e. will not rise in any type of usage, and therefore could not be considered "leavening" because it won't leaven.
There are two types of Brewer's Yeast.
1) Type actually used in brewing of beer/wine/and some types of alcohol. Oh - and root beer too ya'll...
2) Type that can also mean yeast obtained as a by-product of brewing, dried and killed, and used as a dietary supplement for its B vitamin content. (We use this for our dogs combined with garlic to keep the fleas and ticks away - and yes, it works.)