Very cool, if true

The fantasy maps article says, "Tolkien was fine withholding the massive background mythology, most of it wasn’t available until his children decided to cash in on it after the author’s death."

Both parts of this are false.

Tolkien desperately wanted to publish the Silmarillion, which is what "the background mythology" was called, and at one point even insisted to his publisher that it appear as an indivisible part of The Lord of the Rings. The problem was that, as he'd been working on it for decades, the conception had changed over time, and would require massive recasting to fit with the version of the story told in LOTR, recasting he was uncertain how to do and, in the end, unable to accomplish.

Nor was any motive of his estate to "cash in." That's an unjustified insult. The estate was doing very well financially from the works already available. But an audience clamored for The Silmarillion and both the estate and the publishers thought they deserved to get it. What they got, after enormous editorial work by Tolkien's son incompatible with any idea of casually "cashing in," was a kind of photoshopped snapshot of the mythology as best compatible with Lord of the Rings. But that didn't convey the depth of the invention, and by necessity left out much fine work that wasn't compatible. So, after another 20 years of painstaking editorial work, even more incompatible with casually "cashing in," he produced more volumes laying out the whole thing. These have sold moderately well, because Tolkien has a large fandom that wants them, but they're no monster bestsellers, they're no cash cow, and above all they're no instant quickie novelizations gingered up to milk the readership. They're scholarly editions of Tolkien's actual work.