Well it has been confirmed that there is at least a link to ISIS:
Pretty amazing ho fast they caught those two, in the past they had a lot of trouble capturing a culprit that was present at the scene, now they;ve already caught a few guys that used a remote control bomb.
From the article
A letter found at the scene "in the name of Allah", which claims responsibility and reportedly referred to the Berlin Christmas market attack, makes an "Islamic extremist motive possible", they confirmed.
I'm almost willing to bet that they wrote it by hand, it's a rule stupid terrorist make stupid mistakes.