I've been seeing this case pop up over the years. This guy was mining Bitcoin back in 2009, when there were very few people doing it so it was easy to accumulate large amounts. Most cryptocurrency people know this story and use it as a cautionary tale of what not to do.
The way Bitcoin is stored they are not actually on his hard drive but rather attached to a digital address on the blockchain. Think of it like your email, you go to the email website, type in your email address and password then you have access to all your emails. If you were to get a different computer you don't lose your emails, you just need to know your address and password to log back in. It would be a problem if instead of logging in every time you bookmarked the email page and had your browser autofill your email address and password so that you didn't need to worry about remembering that information. If you don't remember/know your login address and password then you are essentially locked out of your email unless you can use the exact computer where you saved that information. Thats essentially what this guy did, he stored his Bitcoin account credentials on his hard drive and never wrote down or saved that information anywhere else.
Of course with email you can always send the "I forgot my password" message to their support and jump through a few hoops. One of the selling points of Bitcoin is that nobody is in charge of it so nobody controls it. There's no Bitcoin helpdesk to call if you forget your password.