Alba has played 40 to 50 games a season for almost 10 years. Pique, Busquets, Xavi, Suarez, Messi, Alves, Neymar, Iniesta amongst others all did the same for extended periods of time.
Dembele, hasn't been able to get anywhere near that and it is down to the training regime.
According to the doctor that Barcelona sent him to, yes. And Dembélé has had three long term injuries, one grade three hamstring tear, one hamstring avulsion and a knee ligament tear, your point would make some sense if we were talking about niggling injuries that can be caused by diet or something like that but not the type of injuries that he got.
Now regarding Alba, he is one of the players that have had to deal with dozens of hamstring injuries, he constantly has to deal with it and play through it.