Myth? Hardly. I specifically asked for stamps with a religious image. They had to go into the back of my local post office to get some.
Several years in a row the "new" Christmas stamps included the Madonna either in painting or sculpture, etc. There have been angel Christmas stamps, Santa Claus stamps, Christmas stamps with new toys, old toys, etc.
On your next trip to the Post Office, ask the clerk what the new commemoratives are since October, if they still have them, that is. Casual amateur stamp collectors often buy them up in bulk and your local Post Office may be sold out. Sold out does not imply religious discrimination.
Perhaps they were out of the Madonna/Child stamps in their drawer and had to get more. (I've had that kind of thing happen when buying non-seasonal stamps.)
Not true in my post office. I asked for the Mary and Jesus Christmas stamps and the clerk took them from the stamp drawer right at the counter (without expressing any anti-Christian sentiment whatsoever, imagine!)
I worked several years as a window clerk in a Post Office. Going back to the back to get stamps is not unusual. Especially at Christmas when there's a high volume of business. The counter storage is limited. The main stamp stock is stored in safes in the back.
The thing I find funny is that the original article assumes an ideological preoccupation on the part of postal workers which I just didn't see in my time at the PO. I guess we just got jaded by dealing with a multitude of stamps. The prevalent opinion was a stamp is a stamp. Some we liked more than others, but mainly your job is to sell what they give you.