Lorn is right.
Reversions are mutations back to whatever the original form was. Since lily cultivars are not produced through mutation from an original form, lilies cannot revert because there is nothing to revert back to.
Original form → mutates → produces mutation → reverts → produces Original form
Many hosta cultivars are great examples of mutations (not reversions), since many new cultivars come from a mother plant that grows (mutates) an offset different from the rest of the plant. A variegated blue and yellow leaf offset from a normally blue leaf mother, for example.
Now if that blue and yellow offset then grows on, and part or all becomes all blue again, but a different blue from the mother, that would be another mutation.
But, if that blue and yellow offset then grows on and becomes the same blue again, that would be a reversion.
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All this said, there is only one unbroken rule in the natural world: There are no unbroken rules. Nature continually evolves, but at such a slow pace that we seldom realize. In the unnatural world, lily mutations have been exploited in through forced chromosome doubling (i.e. tetraploids). Though I've never heard of any spontaneous changes back to a diploid form. If there is such a case, that would be a reversion. I doubt such a facebook claimant would even understand the concept, so the idea is inconsequential to the discussion.
