I would love to believe this but given my experience with my cat even single words are tough for them. For example my cat knows how to fetch her favorite toy but doesn't respond to the word "fetch" at all even with positive reinforcement (treats). The word "fetch" just doesn't mean anything for her even though I've paired it with the action of her fetch many times we play.
The thing with cats is they can do a lot of things, but that doesn't mean they will.
My cats will regularly come when called, if they feel like it.
But... what's she going to do if you say fetch? If she wants to fetch, she'll bring you the toy and if she doesn't want to fetch, you telling her that you would like to isn't going to change her mind.
Merely an anecdote: I had one female house cat that clearly understood a number of words. She could easily and consistently pick out "catnip" in a sentence. "Cow", "get up", "tuna" and several other words and phrases were all understood.
This is unique in my personal experience. I've haven't seen this in other cats.
I’m afraid you’re conflating understanding with obedience.
Treats are good and all but you will probably get better results with clicker training and splitting the job into simple parts. Fetch is pretty complicated and even dogs often have issues with it - our dog did just fine on the 'chase the toy' part - mostly due to strong prey drive - but then just ran away with it :). Took months to get it working.. Also cats usually lose interest quickly, so short sessions repeated more often will give better results.
Anecdotally, the pitch/tone/voice you say something seems to matter more than the words themselves, to cats.
I think you might be conflating cats understanding and cats responding. When cats choose to engage they can be quite intelligent and take direction, I’ve seen it many times. But most of the time they DGAF and will ignore you.
Unlike dogs, cats aren’t seeking your approval. It is a different kind of relationship.
I don't know about cats (I haven't tried training) but my dog definitely knew a few nouns and verbs. She understood "food", "water", "walk", "bone", "ball", "bear" (her toys), and could distinguish between "point", "fetch", and "drop". With "fetch ball" she would go get the ball, whereas with "point food" she would point (paw) at the food, and with arbitrary combinations of these verbs and nouns.
It's astonishing, I didn't think they could do that, but apparently they can.