Your deposited DAI is earning yield, but that yield is not credited as DAI, instead it increases your borrow limit. Example: you put 100k DAI into the vault, your immediate borrow limit is 50k. At an APY of 30% you'll come back a year later and find that you now are able to borrow 80k even though your collateral hasn't increased.
Now the thing worth noting is that the increased borrow limit doesn't count as debt. So if you were to take a 30k alUSD loan now, it won't count as debt.