Apparently they don't think that competition in the market will keep prices as low as possible. Increases in circulating money supply do not automatically turn into inflation/increased prices. Relative to the money supply, when producers can meet the increase in demand, there will be little inflation. I'd argue we have an excess in global production capacity that can quickly be mobilized to accommodate a large increase in demand, thus leaving inflation modest.

It also hugely depends on the competitiveness of the market in question. Corn is not likely to go up in price but your internet bill probably will.

Indeed. One of the issues with the modern economy is LACK of competition. There is significant declines in competitiveness across industries (e.g., https://www3.nd.edu/~wbrooks/BKL_merged.pdf). For UBI to not result in increased prices, and to be effective, we must also solve issues related to monopolization and price fixing of markets.