The assumes financial excess. The reason for UBI is that there isn't excess. If 78% of Americans are living paycheck to paycheck, then that's not excess. If 63% of Americans can't afford an unexpected $500 bill, that's not excess.
You're right that inflation happens when there is excess, but given these stats, I don't buy that part of the argument. Even if these stats could be explained away from being financially irresponsible (I don't believe this is the common case), it still wouldn't be excess because people are still spending. If we don't assume excess then this argument becomes as silly as saying that giving people jobs with an income will cause inflation.
So where could this money be captured? A person that couldn't afford a car can now. I person with a $1k beater car can now buy a $5k car. Someone that was living paycheck to paycheck can now take a breath, buy a TV and a Netflix subscription. Someone that was unable to invest before could use that money to invest. The money affects everyone in different ways, but supporters of UBI think it is clear that there are already avenues to "capture" this money without inflation and that different income groups will spend the money differently. Studies have also shown that when there isn't full market capture cash injection doesn't cause meaningful rises in inflation.