I think part of the concern is that aldehydes in general are not easily used by bacterial species and there’s plenty of other food in our tanks essentially meaning that OP would be adding a potentially dangerous material to carbon dose and wouldn’t really be carbon dosing since most bacterial species in our tanks would not readily use an aldehyde as a food source.In freshwater (where OP question is from) gluteraldehyde is marketed as a "CO2 source" by API, Seachem etc.
Because it usually doesn't kill vascular plants and kills off nuisance algae, and breaks down over time, it's promoted as "increasing plant growth" for planted tanks.
It's a pretty deceptive way to describe an algaecide.
The "food for bacteria" is just stating the fact that it is a carbon-containing compound that is degraded over time. So are a bunch of other things. I'm sure many poisons and pesticides decompose over time, so you could call them "food for bacteria" if you really wanted to, but that'd be pretty misleading.