What "experts" is he referring to?
I don't know, but I do know there are experts out there who can agree with the quote below or at least two out of three of them.
As far as a wall is concerned, the experts actually say that in some places a wall is necessary, in other places that a double wall is necessary, and in other places that a wall wouldn't help.
The first expert to look into (quoting from PBS): Robert C. Bonner, who served as commissioner of the U.S. Customs and Border Protection under President George W. Bush:
“Physical barriers are not a strategy. They are part of a strategy,” Bonner said.
The terrain of the U.S.-Mexico border, which spans nearly 2,000 miles, is diverse. A fence might be the best deterrent along some parts of the border, Bonner said, while other areas would be more secure with sensors and an increase in border patrol agents.
The second expert, Michael McCaul, who was the chairman of the House Committee on Homeland Security from 2013 until 2019. He wrote a report titled 'Blueprint for Southern Border Security' in which he looks into different points along the border, noting different existing measures and recommending different measures to be taken in addition to the existing ones.
A quote from page 4 of that report showing that he thinks it works in some areas and it doesn't in others:
The border fence in the flat open areas of San Diego and Yuma Sectors,
for example, effectively discourages large groups of immigrants from simply walking across the
border and provides Border Patrol agents greater time to identify and respond to threats in those
areas. It is not however, a panacea that will work across the entire border.
The report also multiple fences (using the terms secondary and tertiary) but it does not comment on how effective they are. Aside from that, I think it's an interesting read, it's not very technical at all and easy to digest.