I like to think of a breach as hole through into the hull... they don't mean the boat will sink or even ever will sink; just that the layers of security protections has been compromised.
In the case you mention it seems that happened too: internal actors could reach plaintext passwords and thus for safety the company responded by forcing password reset and disclosure (commendable as I know of companies that would not).
The term "personal data breach" is useful because it defines the range of breaches that the law focuses on (it's not interested in business data or incidents where the first layer of defence fell but the second kept it secure).
I feel it's a bit like having a determination for "road traffic incident". It helps the public, police, etc identify what is in scope... just because you have one doesn't mean you'll lose your licence or be fined - that depends on a range of factors regarding the lead up to the incident: what happened before, during and after. Similar with data breaches.
If a company has a breach it does not mean much in GDPR unless other factors are considered, so I wouldn't worry about being too focused on the term breach.