They can't just pay $X less and hope to still have people working for them though, unless the resulting wage is high enough that the employee will be paying most or all of their UBI back in taxes, in which case the $X reduction is mostly or completely just a regular pay cut.
That doesn't really address the point ? Even if someone is being paid relatively a lot, the UBI still represents a subsidy to their employer who will be paying them roughly the equivalent of the UBI less than they would be if it didn't exist.
A job guarantee relies on there being jobs available, which as we've established is kind of the problem. I guess you could invent some pointless work for someone to do, but forcing them to spend a significant chunk of their time doing meaningless busy work doesn't strike me as being better than not forcing them to do it.
There is arguably plenty of work that is not so much "pointless" as not particularly profitable. Someone to help little old ladies on and off buses, for example. Or more teachers. Or take back all the jobs around publicly funded services that have been privatised and improve it (eg: cleaning staff).