While I unserstand that Pep likes his fullbacks to play like CMs sometimes, I believe he is tactically flexible enough to try different things. Look at Mendy for example, he provides width whenever he plays and plays like a full back.There really is more to this. In Liverpool's system Robertson and TAA provide the width. Mane and Salah tuck in forcing the opposition to get narrow thus the fullbacks having time and space to whip in crosses. Our fullbacks especially Walker are defenders first. Our dual 8's in one of the Silva's or DeBruyne act as #10 and link with the wingers to go as close to the byline to cut across balls for tap in's. Walker has been providing more width, but it's only because of DeBruyne's absence. Our fullbacks are more holding midfielders/defender types. We need fullbacks to fit our system. To me these stat is similar to saying Liverpool's midfielders don't score enough.
I do not want us to gloss over those stats. I love Walker but he gets into attacking positions just as much as any fullback in the league but we know that his end product has a lot of room for improvement.
Just imagine having 2 defensively apt Mendys on both sides whiping in low crosses for fun, our attack would become unplayable.