Is a Named Instance more Secure than the Default Instance?
Not really.

The SQL Browser service is always port 1434: can't be changed. So you need to know the port first before you disable (can't resolve it). Clients then have to know the port which can be packet sniffed.

You can make any SQL Server more secure my enforcing SSL encryption, reducing Surface area, network zoning, proper Group Policy and SQL Server security etc. But not by using Named Instances