If someone come and say that Madrid don't have squad depth, their fans will come for that person, and it is a plain truth they need to believe in; moreover, their defense is not as strong as they may expect which is why they are looking for possible solutions before the January transfer windows; but if they continue like this, they will definitely suffer at some points because some players will surely get injured.
I'm not sure you know Real Madrid defense for you to say they have weak defense system. As long a Mendy, Rodrigo and Alaba are still healthy and not injure like the rest or the defenders, Real Madrid are good and will fight in any match they have unless they are overwhelm by the team they play against. As long as the defenders will do their jobs and forward do the same, they will have chance to win.
I will have to agree with the post you quote because I also see them not having depth on all areas, Madrid have pack of talents upfront and also in the midfield which to me is the their biggest Arsenal. But looking at the defense they seriously lack depth which is due to the number of injuries they have encountered, Alaba is out leaving them with just Rudiger and Militão as the starting center backs, any injury to any of them you are back to playing Touchmeni in that position. They also have the right back position still without depth for many years now Carvajal who was their only natural right back is now injured. Since last two to three seasons now it has been on the news that Real Madrid needs to sign a right back but they weren’t bothered because Dani hardly gets injured and now with him out they and Nacho who was the club’s utility play has left and they have only Varsquez left as the right back or they use Vallejo. So with this I think we can all se that the depth is shallow and not deep enough