The guide states that Spiritual weapon is compatible with Divine Smite and later Improved Divine Smite. What is that statement based on?
As far as I'm aware both Divine Smite and Improved Divine Smite state they are related to melee weapon attacks and spiritual weapon stats that it is a melee spell attack. Rules wise, those are different things.
"when you hit a creature with a melee weapon Attack, you can expend one spell slot to deal radiant damage to the target,"
"Whenever you hit a creature with a melee weapon, the creature takes an extra 1d8 radiant damage."
"When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon."
Happy to be proven wrong though.
Whoops, you're absolutely correct. I was hung up on it being a melee attack and forgot it specified weapon as well. This doesn't change the spell's rating, but I'll update the description.