This article leaves more questions than it answers. Room-elephant number one: access being available after an employee has left is bad. That access remaining five months later is beyond the pale, unless the real story is that the employee created a backdoor. Barring a backdoor, there are further serious questions about the employee retaining this access, presumably without any employer-provided and controlled hardware (e.g. laptop, yubikey, or what-have-you).

Room-elephant number two: motive. The reported facts naively summarize as "oops, ex-employee blew up some stuff in prod, caused problems". <meme>But whyyyyy??</meme> There's no indication of specifics, and seeming denials of some obvious guesses: attempts at hacking (e.g. data exfiltration for profit, which are denied), ransomware, revenge, or anything else that would explain this behavior.

Further confounding everything is the bit where the new employer's response to these revelations is apparently "shrug".