Media has reported their cartoon as 'Mockery of Muslims'. When you mock an entire religion, you are being racist against them. Even more, when you add a photo of two naked Muslims and then write a witty comment about 'Muslims', you are suggesting that is how Muslims are [even if you are trying to say it is stupid to believe a ban on Burkini will make the Muslim act like that]. Because many Muslims don't even wear Hijab, let alone Burkini, and I know many of them who do actually go to nude beaches.
Charlie Hebdo is not a Muslim magazine. The Media is not using their satire to mock Charlie Hebdo, but is reporting it as a mockery of Muslims. The events aren't similar.
Note that my argument for Charlie Hebdo being responsible for this is that they promote racism, and that racism promotes extremism. Similarly, Muslim extremists are responsible for the racism against Muslims.