I read the same thing, but I don’t quite know what to think at this stage. Have a look at this article from 2010, which implies that this is established:
Metformin-induced Vitamin B12 Deficiency Presenting as a Peripheral Neuropathy
But this 2014 article from the Joslin Diabetes Center presents a less absolute view:
Should You Take B12 Supplements if You Take Metformin?