He said he would strongly recommend getting tubeless tires.
How many flats have you had in how many miles? How many flats has *he* had in how many miles? What advantage does he imagine tubeless tires gives? Clearly a tubed tire has two layers to keep air in...
Given that this is a cross country type trip, you may even find yourself on dirt or gravel roads where the puncture probability goes up. If you had to change a tubeless tire, are you bringing a spare tire? Patch kit? Rim lube and tire levers? Air compressor or co2 cartridges?
Certainly flat tires were more prevalent in the past, I would argue that tire technology (aside from being tubeless) has improved quite a bit and reduced the chances of a flat.