The following is just my opinion...
This is the first time that I have heard this one. I personally wouldn't put much stock in it. Clay has been used in shaving soap for a long, long time. Sure, bentonite clay is gritty and that can leave scratches on the bevel of the razor, but one of the main reasons for adding it to shaving soap is to improve the skickness of the soap. So, it should actually improve how the razor glides over your skin rather than inhibit it, which I think would actually help the blade last longer.
Keep in mind that a just using a straight razor to cut hair (i.e. shaving with it) will dull a straight razor. Straights have delicate edges (be very careful not to drop your razor or ding the edge on any part of the sink), that is why they require maintenance. Stropping before and after the shave will make sure that your edge is in the best possible shape for shaving. However, every shave will degrade the edge if only ever so slightly. Using good technique with the blade will help the edge last longer, but at some point (which could be after weeks or months of regular use) it will require re-honing. Using the blade for its intended purpose will dull it faster than any ingredient in your soap.