Boko Haram like other Islamist groups like ISIS are consistent. You need to understand what Shekau is saying before you stan. No, don't be silly, he isn't ridiculing the sentence; far from it.

Blasphemy against Allah as punishable by death is a very mainstream view. The extreme is how wide this definition of blasphemy is willing to be stretched. For many, it isn't just blasphemy against Allah per say but his also Prophet or his word (the Quaran) that is equivalent.

There is also a widening of definitions of what acts are considered blasphemous. Insult, mockery, jokes, praising someone else more than Allah or his Prophet... Following a law other than God's law... Being subject to a government system other than God's Caliphate...

... Educating yourself and children with other forms of knowledge asides Islamic...

For Shekau, your average Nigerian Muslim by being subject to the Nigerian state is blaspheming. Which is why they are fair target for killing by Boko Haram.

Know what you are stanning.