Near the beginning of my career, I talked to a greybeard who harrumphed at me discussing something-or-other and said "computer security is an oxymoron". I thought he was being too pessimistic, nowadays I realize he was right.

That's too pessimistic. But it is a spectrum. You can't guarantee 100% success against 100% of potential attackers, but it still matters how easy it is to get into something. There's a pretty big difference between a Windows 95 machine hooked directly to the internet and something like a fully up-to-date iPhone. The iPhone is still hackable, but in practice it's so difficult that you're unlikely to be targeted unless you get the attention of a national government.

It also requires actually caring about security and putting effort into it. These data breaches are usually systems where little attention was paid to security in the first place, and e.g. getting ahold of one user's password is enough to lose the game. Getting companies to care about security is really hard, but it does happen.

If he's grey enough, he might have predated effective cryptography.

Real computer security IS possible but takes a lot of effort by very skilled and dedicated people. You don't hear about bank mainframes getting hacked often.

it's silly to think about computer security as binary secure/insecure.

Human security. Computers are fine, they usually do exactly as they’re programmed.