I also had this happy surprise happen to me too. If you click on the [ >| ] button to expand the other printings, you'll see that Boom Tender is available in the Eventide printing at rare or Special Guest also at rare.

I agree with the policy.

It would be extremely helpful to have SOME sort of information prompt explicitly declaring it for this circumstance as the interface is prima facie contradictory.

That IS correct. For a year now, due an intentional and announced change to the crafting process, the lowest rarity printing of a card on Arena determines the wildcard rarity nessessary to craft any version of the card, regardless of the rarity of that version. Bloom Tendfer's lowest rarity on Arena is rare, so a rare wildcard is needed to craft the mythic version from Lorwyn Eclipsed.