Although there may have been a single architect or a group of architects behind the initiation of these projects, the software design has evolved over time by being touched upon by multiple contributors. In way, these systems have lost their fragility.
Given enough eyeballs, all bugs are shallow
Or more formally:
Given a large enough beta-tester and co-developer base, almost every problem will be characterized quickly and the fix obvious to someone.
Both the above statements point to the fact that problems which otherwise may escape a centralized team of architects and developers may be found and fixed when the code is exposed to multiple eyeballs.
So the design of the software evolves over time in a natural, organic fashion, which makes the system antifragile.