The current default is true - you can see it in config/defaults.yml
But that (‘true’) is what is the case when you uncomment that line to make it active. It doesn’t tell me what is the case when that line remains commented out. To me, the fact that uncommenting that line activates jquery_cdn: true suggests logically that, when that line remains commented out, the value is false.
From the introduction to diaspora.yml.example:
All settings are by default commented out with a single #
You need to uncomment them in order to work.
If what you say is true, that text needs amending!
Well no not really, since all settings have a default. Those defaults are in config/defaults.yml if no other is defined.
Yes, but then by uncommenting the lines you’d just be keeping things as they would be if the lines had been left commented, which makes no sense to me.
It would make more sense to me if, in this example, the line to uncomment to activate read jquery_cdn: false, and the accompanying text read something like:
## jQuery source
## By default, jQuery libraries will be obtained from jquery.com's CDN
## This potentially saves you some traffic and speeds up
## load time since most clients already have this one cached
## Uncomment this line if you want jQuery library to be loaded from your pod's own copy.
#jquery_cdn: false
Yes, I think you’re right! 
Have just seen that the default settings are given in, err, defaults.yml (sorry, didn’t clock that from your earlier comment as was rushed), so now I’m clear on what the case is.
In fact I’m going to try to clarify many of the explanations in diaspora.yml,example at the same time.
Still think it’s better to leave the actual changing of the default setting until a major release so that podmins don’t get caught out.