Marvel and DC Comics must have easily exceeded that number since the late 1930s, when they both began publishing stories which would eventually become part of their respective universes. That's even discounting the many licensed titles they have published over the years featuring characters which they don't own (just as ACP's figure of 25000 doesn't include the Archie Adventure/Archie Action titles like TMNT and SONIC). That's because the number of new (discounting reprint) comic book story pages that those other comic companies publish in any given month in their history has always exceeded the number published by ACP by factors of 2 to 50 times as many, so it more than balances against the fact that the majority of comics published by those companies since the 1980s have featured only a single story, instead of the 3 or more you'd find in a typical Archie Comic.
That estimate of 25000 they're giving isn't just for stories featuring Archie, because the title of the book is "The Best of Archie Comics" not "The Best of Archie" (the character). If Marvel or DC Comics were to publish a similar book series featuring the best of the company-owned stories from their 75+ years' history, they'd both have a lot more stories to choose from.
That estimate of 25000 they're giving isn't just for stories featuring Archie, because the title of the book is "The Best of Archie Comics" not "The Best of Archie" (the character). If Marvel or DC Comics were to publish a similar book series featuring the best of the company-owned stories from their 75+ years' history, they'd both have a lot more stories to choose from.