What exactly does "hijack" mean though? I don't speak IRC so it's not clear what they're actually doing to the channels.
Hijack means removing those privileges from someone else and assigning them to yourself.
In Reddit terms, imagine the Reddit administrators coming to your subreddit, deleting the moderators and then installing their own.
Administrators always had this power. Sometimes, this power was used out of necessity. But given how much this power was used in FreeNode in the past week, a lot of the community has lost faith in the administrators.
But yeah, if you control the server (the nature of being an administrator), you can do these things rather easily.
To be slightly more exact, the admins suing those powers properly for the past couple decades have been pushed out by the new guy, and said new guy has already abused those powers so much in a single week that nobody will ever have any faith in him
But that's probably an anachronism, given the age of IRC.
Now you've moved to another place but have no way to leave a signpost or redirect. And anyone who doesn't get that memo through some other means, will land in a confusingly-named place that looks official but is precisely the opposite.
I'll not go into more detail as existing threads here cover it far better than I could.