The article's sources disagree with the article. Its link to Microsoft's definition of a security boundary explicitly includes containers as a security boundary twice in the tables and offers bounties if you can break out of that security boundary. Its link and quote from Google say it's not a _strong_ security boundary yet the article claims Google said it wasn't a security boundary at all. The Red Hat link doesn't say anything about security boundaries whatsoever but it does say containers aren't perfect protection yet they do provide some protection. The Netflix link also explicitly says containers are a security boundary multiple times and they use additional protections to strengthen that boundary. At this point I'm doing following citations but you get the point.
If the security folks at your job truly doesn't consider containers security boundaries then they are wrong. What seems more likely is they don't consider containers alone a _good enough_ security boundary. And that's fine, some places consider separate processes with different rights good enough security boundaries. Others consider two boxes that are able to interact with each other not a good enough security boundary. It doesn't change that things that weren't secure enough for the use case are still security boundaries.