Only fantasy football has WR1, WR2 and WR3 positions. It doesn't work that way in the real world of football. That makes as much real world football sense as saying Anzalone was drafted to be LB2.
Matthews was singed to compete as a WR on the outside and/or on the inside depending on his strengths and weaknesses. He may end up being the second best receiver on the team, but he wasn't signed to be WR2.