On stackexchange.com/tour the sample question is very unscholarly and hypothetical. I don't think it is on-topic on any community here.
Should we have something like that on the tour page?
The question was changed on other domains.
TL;DR - can't see any bug or problem here.
stackexchange.com is the portal of the whole network. This is not a Q&A site on its own.
So, being "on topic" is not relevant there.
As can be seen here, unicorns are a very popular meme in Meta Stack Exchange, hence in SE in general. So it just fits for choosing as a sample question which should not target any specific site.
Each site can choose its own sample questions (i.e. the site moderators can), so really can't see any problem with this mechanism.