Mmm, that is a highly debatable contention at best, honestly. Certainly Tolkien wanted to publish some version of the Simarillion at various times - the first as early as 1937, before even the Lord of the Rings - but it was rejected twice and he shelved it and worked on LotR instead. He then changed his views on the backstory of Middle Earth significantly through the 40s and 50s and didn’t really have a final publishable version available before he died. The 1977 version, which is mostly what we have now, is half him and half his son Chris and Guy Gabriel Kay working on his later notes. Chris Tolkien admits that he wrote much of it from scratch and would probably have written a different version at a different time. It’s necromancy.