Only going off you tube videos so I know not conclusive but there are lots of examples of some very good passes cross field to switch the play. What makes you think Pep would buy a full back that can't pass and therefore give possession away. It make no sense, after all the full backs he has worked with over the years.
Not being good at passing =/= giving the ball away. Fernando isn't a good passer of the ball, but he has a 98% success rate. He doesn't give the ball away.
I think Pep would want him because he appreciates work off the ball more than anyone, he values someone who is positioned well, who makes intelligent runs, who can win the ball back quickly and decisively. He knows we desperately need someone who will defend half the pitch on his own, who will cut out counterattacks and actually go toe-to-toe physically with anyone.
Not only that, but his team are on the record as believing players can improve their basic technique no matter how old, and use the example of Abidal in Pep Confidential as someone who was a great physical defender but technically limited when Guardiola arrived, and was completely transformed by the time he left, despite starting that transformation at 30. He was taught better control, passing, and how to read the attacking side of the game, and he went from good to absolutely great, and a vital part of Pep's Barca.
So I think Guardiola will see the athlete Walker is, with the 1v1 skills which make him a great defender, the intelligent pressing game and the progress he's made tactically under Pochettino and think that he's going to hold down the RB position nicely as he is, but also that he's a player who's capable of learning and adapting, and with some improvements on the technical side he can reach a whole other level, like Abidal did.
I'm excited by the signing, I don't want people to think I'm not just because I picked up some technical deficiencies in his game, I just think he's very effective despite those and if anything that means he's got room to grow which makes me even more excited by the signing.