RFID-Safe Wallet destroys mag stripe?

Short answer: No.
Long answer: Nooooooo.

Longer answer: RFIDs typical work via magnetic coupling to both power and to transmit data. By modulating the load on the secondary coil of the magnetic couple (aka transformer), the RFID card is able to effect a back EMF on the primary coil.

RFID-proof wallets disrupt this by creating a Faraday cage (i.e. opaque box to RF) which prevents the magnetic coupling from occurring.

In order words, it is literally a tinfoil hat.

Seriously. You can build your own RFID-proof wallet by putting tinfoil in it.

What can happen, and actually had happen, is that some card holder were badly designed and included a simple magnet for locking.

This will do nothing to the RFID part of the chip, but it can damage the magnetic strip that is used in some terminal (The ones where you slide the card and don't have to enter your pin code).

Note that this is not limited to "RFID safe" wallet, but can happen with any magnetic locking wallet if you get the strip to close to the magnet. This was also known to disable some bus ticket or other magnetic cards.