It seems pretty far to me. Yvain brings up two examples at the start of the article:

I think most decent people would be willing to go to some trouble to avoid taking pictures of salmon if British people politely asked this favor of them. If someone deliberately took lots of salmon photos and waved them in the Brits' faces, I think it would be fair to say ey isn't a nice person. And if the British government banned salmon photography, and refused to allow salmon pictures into the country, well, maybe not everyone would agree but I think most people would at least be able to understand and sympathize with the reasons for such a law.

Does this apply to you? The "politely asking" case (case 1) brings a lot of sympathy from me.

The second case, "thou shalt not...", I understand, but I now have less sympathy.

The third case, I think, would be if the Brits forbade all photos of salmon, anywhere in the world and tried to enforce the rule worldwide. This still doesn't seem to get us to "you must believe as we do".

To get to "you must believe as we do" (case 4), the Brits would have to steal all our children and raise them to experience as much pain as they do upon seeing a photo of a salmon.

The slippery slope seems to lean the other way, such that the Brits (or Muslims) need to stay tempered. The closer the they get to case 4, the less sympathy they have. At case 3, all of my sympathies are gone, and I would be tempted to take photos of salmon and leave them lying around just for fun. At case 4, I'd consider fighting a war.