You might just be gifted with great memory. I‘m not sure working memory can really be trained in a significant way. Happy to be proven wrong of course.
One pretty strong argument against that claim comes from an interesting experiment with world class chess players. When you give top players positions from real games they can often memorize dozens of boards, which makes for pretty impressive blind simultaneous matches, I think the world record is in the dozens. But when you hand them illogical or random positions that never occur on boards they can maybe memorize one or two boards, only a little bit better than an amateur with reasonable memory.
You could even try this with programming. Have a programmer memorize a page of code then try it with someone who has never coded. Pattern recognition, which is learned, has huge impact on ability to memorize. It's sort of intuitive why, if I see three functions and I can recognize that it's fizzbuzz, recursive fibonacci and bubble sort all I need to remember is variable names. My dad who has never seen code would have to recall every symbol.
It is a lossy form of compression. You dont remember exact copies of all of your sensory inputs - your brain abstracts and composes, at every level of abstraction.
They also mentioned a lack of distractions, allowing them to grow their attention and focus. I'm pretty sure all of those will affect one's memory.
Sometimes the wonders of modern computers are as much of a trap as they are beneficial. It was far easier to separate yourself from distractions when the tools were not a source of distraction in their own right. If the television was a distraction, you simply turned it off or went to another room. You can't simply turn off a computer or walk away from a computer when the task you are trying to complete is on a computer. It also helped that computers weren't quite the communications and entertainment devices that they are today. That is especially true of personal computers until the early 90's, where one had to quit a task, rather than momentarily switch away from it, to do something else.
If you have that attention or focus, modern computers are absolutely amazing productivity tools. If you don't, it is an uphill battle to stay on task. Older computers may not be nearly as amazing as productivity tools, but at least you didn't have to face that battle.
