UBI is not the first time someone has proposed that a central authority should determine the income of the general population. It doesn't take much research into Russia to recognize this as 'serfdom'.
original source of this example: https://ploum.net/largent-doit-il-etre-notre-seul-objectif/ (in french, but interesting essay)
The original argument was that GDP would decrease if we had less people producing, the counter argument seems "we could make GDP artificially increase" presumably as a way to suggest that GDP isn't meaningful. That's not a very good argument because artificial GDP inflation is a temporary solution that has devastating results down the line
>Increasing the GDP is easy. First, pay me 50$ to dig a hole. Then I pay you 50$ to fill the very same hole. We created nothing but the GDP increased by 100$ (well, tax aside).
there's nothing about individual income there.
Briefly, the only possible reason why there would be inflation is that the people given money for digging and filling holes now have more income that they can spend, so they increase demand for products. If this raises the price of products (and yes, that's an if), it will also mean that companies will have more money and an incentive to hire new people so that they can meet the increased demand. Some of those people will come from the digging-and-filling-holes program. It's a self-stabilizing feedback loop.
Obviously there are more subtleties, e.g. when a lot of domestic consumption is from imports. But even then, you wouldn't get runaway inflation. In the extreme case, currency markets would adjust up to the point where it becomes profitable to increase domestic production, and you'd get self-stabilization in that way. And increased domestic production in itself would be seen as a good thing by many people.
As for the whole Keynes being wrong thing, you didn't actually address what I wrote about inflation and how giving people a fixed amount of money for work (even if it's "make work") is self-stabilizing.
Also, to your "more money in the system than the system is producing": Keep in mind that the amount that the system is producing is not a fixed quantity. It's variable and part of the whole dynamics.