2) If he can defend, he'll play in more big matches than Dembele - I assume. But we don't know that. All we have is his statement. On the other hand, defensive contribution was regarded as his main weakness in Bordeaux. I read somewhere that back in Brazil he worked hard in both directions, but since he was Bordeaux's main star didn't do much. Shall see.
Defensive contribution is often pointed out as main weakness with most young forwards/offensive minded players unless they are workhorses like Pedro who are willing to run all day. Pedro has often even ran too much and was wasting his energy (IIRC even Pep said that to him a few times) but he was appreciated by most fans for always giving his best for the team.
For players with weakness in their defensive game clubs should recognize whether that's mostly because:
1.) lack of tactical discipline/awareness - players are often found out of position because they don't know where to be or if/when to track back because of their inexperience and/or lack of football IQ. If they have high enough football IQ (I believe Dembele does and we'll see about Malcom) then they should improve naturally when they get more experience and obviously with working on their issues with coaches in trainings.
2.) players don't have physical attributes to contribute defensively - this can be a problem with older players (and in our case is with Messi) but usually shouldn't be with young players (most of them are great athletes but of course you also have players who are slow and/or don't have lungs to run for 90 minutes when they are 20 or 23) on top level unless they are not 100% fit which I think was the case with Dembele last year so I think he'll improve his defensive game this season if he'll stay healthy. And with a coach like EV he should if he'll want to play. I don't expect him to turn into Pedro but he'll have to do his share of defensive work like other 8 outfield players. Midfield and defense already have to work harder because of Messi's and many times also Suarez's lack of effort defensively so we can't afford to have another player in the lineup who won't defend when he has young legs/energy to do so.
3.) players don't have work ethic/workrate to help defensively - unless you're Messi you won't succeed at Barça if you won't give a s*** about doing required defensive work to help the team. But even Messi worked harder when younger or he wouldn't become what he is today and get a privilege to "run smart" or in other words not doing anything defensively in 90-95% of the games. But because of who he is and what he brings to the team offensively his team mates are willing to give 10-20% more in defense to compensate for that. They shouldn't do the same for other players.