Nothing indicates that the decimal point error ever was made, but the account about it will most likely live a long and colorful life, just like its parent myth
Spinach is a fictional plant, it was described by Linné as sort of a joke but then everyone just went with it.
But seriously, you described three layers of misconceptions in this comment thread, how is anyone supposed to know "the real truth" about anything food related, if spinach alone is such a hard subject?
Apparently there is no documentary evidence why an 1870 measure of spinach's iron content was exaggerated. The paper cited does not explore whether it ever was exaggerated, or what its actual iron content then or now might be.
I read various reports indicating that modern vegetables have much less of various nutrients than older, slower-growing or smaller-yielding varieties, and have no idea how I might evaluate such claims. Maybe spinach harvested in 1870 had more iron than highly-fertilized 1930 varieties, never mind 2021 varieties no one, to my knowledge, has bothered to measure. Or maybe not.
Nothing indicates that the decimal point error ever was made, but the account about it will most likely live a long and colorful life, just like its parent myth