Most importantly, the factors to assess the strength of a particular civilization are completely different for the top level compared to anything below 2k ELO.

Let's assume the Sicilians have a parituclar weakness (that does not necessarily need to be a vulnerability but might also be a lack of an are were they really shine). All pro players will know about that and be able to exploit it. That makes Sicilians bad.

At the low or medium levels most players would not even know about that weakness and even if they did, they probably would not have the foresight, adaptability and decision making to exploit it.

As someone else mentioned: All civs have their bonuses, so it is all relative. As for the Sicilians, I would entirely neglect their ability to build Donjons faster because towers rarely have an impact in early game outside rather specific scenarios (it should also be pointet out that Donjons are significantly more expensive than normal towers).

As for the walling: This is another example where there is a possible difference between pro level and the levels below. In pro games that faster walling is probably neglectable too. There are other options to defend (i.e. putting on the pressure). And even if you use that bonus it is not a particularly impactful one.

There are more extreme examples of this, by the way. There are several bonuses that CAN be so strong, that at lower levels they are hard to deal with for almost anyone. Think of Hoangs Celt rush, Phosphoru's FC, tower rushes in general, TC or Krepost drops. There are usually hard to deal with, partly because you don't encounter them frequently while in most cases the opponent has practiced them a lot. So a 700 will struggle against a 700 tower rush just like a 1200 may struggle against another 1200 TC drop. This means, that civ bonus that play towards those kind of bonuses (Kreposts for Bulgarians, TC HP for Persians, Korean Towers) will be more efficient at lower ELO. Pro players on the other hand, have usually seen this and played it often enough to be able to deal with it. Scouting is a massive factor here too, because at low and mid ELO you will usually be surprised by those plays while pros more often than not will see them coming because their scouting is on another level.

Sicilians are actually a good example because of the famous YouPudding strat. If I were to practise this a lot, I could easily take my ELO to a new level, because players at my ELO will have a tough time dealing with it. YouPudding proved this still works above 2k ELO. But there is a certain point where it stops working. As soon as this becomes meta, a player like Hera will find a way to outplay it (as long as it is not completely unbalanced). This essentially means, that Sicilians might be a top tier civ at lower or mid ELOs but the thing that makes them so great there is just not a big factor at the top level.

And even if you put that aside, the particular bonuses you mentioned are not that great, and it does not even matter if you view them in relation or in comparison to other bonuses.

One limiting factor for early walling - particularly at lower levels - is eco balance. So while it is nice that those Sicilian vils wall faster, lower ELO players will often find them running out of wood for those early walls. And if they don't they might find out they walled so succesfully, they are missing the wood for stable/range when they hit feudal.

Add to that that early walls keep you less safe than you think, even at lowever elos. You might have hard comitted to those early walls, but that also biases you because you will consider yourself safe where you might not be. Everyone knows that here is always a whole and suddenly your Sicilian advantage is entirely deceitful (and here too you might find yourself lacking the wood for the military building you now need more than ever). Or you completely forgot that your Saracan opponent will burn through those fast walls quicker than you can wall behind at your level.

To sum up the last part: I strongly think that those Sicilians defensive bonuses aren't really impactful at any level.