This is an interesting question, the article that you mention gives a great summary of the technical issues in this change.
Let's look at your question in more detail: "Is this legit and an actual safety concern?"
- Is it legit?
Absolutely the article is technically accurate. In summary by allowing more users to write to a directory that is high up the path it allows for some interesting types of attacks.
- Is it an actual safety concern?
If I may I think you are trying to ask "Should I be worried about this?". The reason I like to rephrase this is simple, there are plenty of issues out there that are legitimate security concerns that people choose not to do anything about. This can range from behaviour (connecting to public wifi) to bugs in software which the vendor has chosen not to fix.
Unfortunately, no-one can tell you whether a particular issue is worthy of taking action over without knowing much more about your environment and attitude to risk.
For example, I am ok (on my personal MacBook) to keep home-brew installed on the system with the default options. Why? Put simply the benefit I get on a system I use for development from having brew installed outweighs the risk.
Please note, this is my personal choice and not a recommendation for you!
If I was running a Mac based shared server I would be less comfortable with this, I would be trying to lock down as many accounts as I can and preventing most users from being able to impact global settings.
In summary, you need to consider your (or your organisations) attitude to risk and decide for yourself whether the changes that homebrew make during a default install are acceptable to you or not.