I think the rationale is that between UBI and his plan, his has more upside. I don’t think AK is a good analogy because, as you state, it’s a nearly annual re-supply of cash
1) not everybody will use it because they won’t want to do the civil service part
2) those who do won’t have perpetual payments. Once you use it (unlike Alaska) it’s gone. If you squander it, at least there is an upper limit to your benefit as opposed to a plan that acts as an open-ended annuity. I actually think this may help foster better decisions because for many, this is their one chance at such a windfall. Of course there will be some who squander it regardless
3) As opposed to UBI, the rest of society will get some benefit in the form of service.
To elaborate on the financial wisdom aspect, he advocated for putting the money in a trust fund payable at around 25 years old. Similar to the GI Bill, you could also have strings attached as to what it could be used for (the GI Bill, for example, can be used to open a business, go to university, go to a vocational school, but not just a check for “free” money)
