It's the first you've heard? It's been a GOP talking point for a while now. If anything, Mueller know Trump is guilty of obstruction, but that's not the focus of his investigation and discussions like this assist opposition to Mueller.

The mandate of Mueller's investigation is coordination with Russia during the campaign, and any illegal activities that surface during the course of said investigation. Read the mandate yourself. https://www.justice.gov/opa/press-release/file/967231/download

To speak as if obstruction is the target of the investigation is to be absurdist and distracting. This entire article is thinly veiled, anti-Mueller propaganda to muddy the waters around the investigation.