"Whenever you summon" rule

Suppose "Thrall, Deathseer" transforms one minion into Unlicensed Apothecary as shown here:

See: https://hsreplay.net/replay/Vx26aE6bsHz4QuTDUnkqPi (see the end)

It will (for reasons I don't understand) trigger and do damage to you. Can anyone explain why this is? Are transforms equal to summoning?

If so, would polymorph on a creature trigger Unlicensed Apothecary? What other spells would trigger it?