This is an attractive but rather over-simplified view. The combination of the concentrated salt solution that is the Dead Sea and strong sunshine does seem to be of benefit but the precise mechanism has not been fully evaluated. The seawater also contains some tars that may also be of benefit. Holidays in sunny places can be relaxing and if other people with psoriasis are at the same resort you would be much less afraid to take your clothes off and expose your skin, thus getting more benefit from the sun.
As the Dead Sea is below sea level, the atmosphere is relatively thicker and absorbs some of the UVB light that causes burning. This means that you can get a higher concentration of the helpful UVA light without burning, but this could lead to higher risks from skin cancer. Most of the reviews of Dead Sea treatment mention treatment times of under three hours. It is thought that the complex salt solution adds to the efficacy of natural sunlight and may in itself have some effect on skin cell turnover.
Even the dermatologists from the Ben Gurion University in Israel stress that treatment guidelines and standards have to be set in order to assess the cost effectiveness of Dead Sea therapy compared with other treatments. The NHS has to be very careful about spending money in this way, and many other patients might demand holidays because they feel better afterwards.