There is no way, currently, to hide the News feed channel side panel/column, in the new Apple News app, in the new iOS 12, when using the iPad in landscape orientation.
The News channel side panel/column feature of the Apple News app may not be a bug, but works similarly to how the Apple iOS Mail app panels work with two panels always visible when using iOS/Apple Mail app in iPad landscape orientation.
So, this may not be an Apple News design flaw, per se, but an Apple News feature design change/enhancement.
The News channels used to be a second option in the lower screen menu that you had to tap and then tap to choose your news feeds, then return to the main page in the older version of the Apple News app.
Apple made the news channel panel/column always on and available in landscape viewing mode to more easily access and update those news feed channels on the fly, now.
So, once again, this may not be a bug in the Apple News app, at all, but a feature enhancement.
If you do not like this new feature the way it is, you can complain/send feedback to Apple.
The only way to get Apple to really listen to you, DIRECTLY, is to use their feedback portion of their website.
iPad Feedback
http://www.apple.com/feedback/ipad.html
Currently, there are ONLY a few hundred users ( currently, over 400 users, now )complaining about this Apple News feature/issue in the iPad support communities.
For Apple to affect a change to their News app, there needs to be a WHOLE LOT MORE users making a complaint and sending feedback to Apple about this, like in the thousands, or more.
So, if you want Apple to fix this, perceived, error/oversight/omission/bug, you must tap and post your feedback to Apple per the link I supplied above.
Good Luck!