Yes, but Wikipedia is also frequently conceived and marketed as "the sum of all human knowledge," which that shows is a lie by definition.
https://en.m.wikipedia.org/wiki/Wikipedia:Prime_objective
You seem to have an issue with some person or persons who has been advising others that the only work they need to consult is Wikipedia. Who are they? Specifically.
By which definition? In math, the sum of a set necessarily implies a loss of information about that set, for sets larger than 1. But they're using "sum" not in the purely mathematical sense, more like "the summary of all human knowledge". But the same principle applies axiomatically, because summaries are lossy compression: you cannot have a summary that contains all the information of the source it is summarizing.
> By which definition?
That's pretty easy: definition 2 "the whole amount : aggregate" (https://www.merriam-webster.com/dictionary/sum). That it's interpreted that way is shown by the frequency of people saying stuff like "I loaded Wikipedia onto this battery powered Raspberry Pi in a Pelican case, now I'm ready to rebuild civilization if it collapses," and seemingly believing it.
But you do correctly point to another issue: sum has a meaning of "a summary of the chief points or thoughts," which I feel is a less common usage. So the marketing phrase may not be so much a lie, but rather an extremely misleading statement that invites misinterpretation that usually goes unchallenged. IMHO, those are actually even more pernicious than outright lies.
The least delusional part of this is the sparseness of information contained within Wikipedia. If that scenario came to be, they wouldn't be short on information. They'd be short on time, resources, and skills.
As someone who inched closer into the doomsday prepper scene before swerving far away from it, I assure you that people in that subculture have a lot of unrealistic beliefs about their own capacities and resources. I don't think it's Wikipedia's fault that they (and you) are taking a quote about Wikipedia's never-ending goal and interpreting it as if it is their description of what they are.
An even worse example of deceptive marketing would be a compact folding multitool marketed as "the only tool you'll ever need." Even with that, I'd say that if you actually believe that you can rebuild civilization with that tool solely on the basis of that marketing slogan, then that's your fault as much as it is the marketers.
And a minor nitpick: the standard prepper info archives also include collections of various survival guides and resources that are specifically written for these kinds of purposes.