As ktorster points out, your article is spreading a bit of misinformation.
On Apple ID's, iCloud, App Store & iTunes Store settings...
All of this can be a little confusing since you can use your Apple ID (aka iCloud) as your iTunes & App Store ID but you absolutely do not have to. It's smart for each family member to have their own FREE Apple ID for all of the various services that are personalized to each member of your family. That's the way Apple planned for Apple ID's and iCloud services to be used... by INDIVIDUALS.
If you also want to share an iTunes or App Store ID, you absolutely can do that as well, although now with Family Sharing, it's far less needed to share purchases. For folks just now getting into the Apple ecosystem, I would setup separate everything and deploy Family sharing. It does keep things much cleaner.
Messaging issues... On the iPhone, there are straight SMS and MMS text messages which use your carrier's messaging services.
Unique to Apple devices, there is ALSO iMessage. iMessage is tied to your Apple ID (iCloud) and allows for end to end encrypted communications using your iPhone, iPad, iPod Touch and even your Apple Computer. It's extremely useful but it is completely optional. You do not need to turn it on.
If folks are having their messages showing up on another person's iPhone, verify that you're NOT using the same Apple ID for any of your iCloud services. If you are sharing, you absolutely could get messages echoed to devices you hadn't planned to. I say "could" because, even this is configurable in your messages settings.
I imagine that folks who are having issues with some or all of this, simply do not have their Apple ID/iCloud/App Store/iTunes Store settings correctly configured in their devices. I would urge those folks to contact AppleCare (usually FREE) for help or to set up a FREE Genius Bar appointment at your nearest Apple Store. You could have many if not all of your issues solved in less than an hour.
BTW, I am a cross-platform user & network support analyst with more than 35 years of IT experience, just in case you were wondering. ??