Well you see, a letterbox is uniformly coloured except for an opening, and a burqa is too. So he's using it as a metaphor to imply that they look similar, in doing so ridiculing it. Clearly they're not the same thing, but that's the joke. You know when you dissect a joke it loses it's humour. You can apply the above with bank robbers, they typically hide their face to avoid getting caught.
And that's the thing Alex about both comedy and offence. What you find funny, I might not, and you what you find offensive, I might not. The point is whether or not you find it funny or offensive is irrelevant. The law is about having the intent to incite religious hatred. If you want to persecute him for it, you'll need to prove it. Intent to ridicule for comedic purposes (whether or not you find it funny) is not incitement of hatred.