As far as I can read into it, the idea is to avoid people making a quick cash-out in USD, so the app makes these airdropped 30$ in BTC non-convertible to USD, within the scope of the functionality provided by Chivo. One could, as you say, send it to another wallet and do whatever he wishes from there on.
That's the thing: if it's "limited" money, I would want to get rid of it. But if it's "normal" money that I can use however I please, I'd treat it like normal money. Too bad it takes them so long to get their wallet working, I'm very eager to see what's going to happen.
Sending a small amount of Bitcoin to a normal on-chain wallet sounds terrible for fees though, and if a few million people are going to do that in a small amount of times, fees might go up a lot.
I figure the ruling hypothesis is that most people will stick to using Chivo, at least for a while, and incentivating them to use these 30$ of BTC (as opposed to cashing it out), even if it by sending it to another wallet, getting the BTC ball into circulation. The expectancy nevertheless, I figure, is that people get to use it in some sort of everyday life commercial TX.
I assume Chivo is custodial, so there's a "restore" option if people lose it. If this many people would suddenly have to be their own bank, they're going to need the time until September to educate them on wallet backups.