Are you sure that's right? The actual wording says, "you can use spells slots you gain from the Pact Magic Class feature to cast spells you know or have prepared from classes with the spellcasting feature." That explicitly states the spell slots remain warlock spell slots. The issue is actually that the font of magic class feature applies to any spell slot. It just needs to be limited to sorcerer spell slots or spell slots gained from the multiclass spellcaster spell slot table, which would exclude warlock spell slots.
And it explicitely say, what you can do with them: casting spells.
What is not stated? That you can use them to fuel class abilities that rely on spell slots.
So it is TRDSIC, not RAW that you can use warlock spells for smites...
and then, it is only a few more d8's per day, which you pay with later access to high level powers like improved divine smite.
Multiclass combos just peak at certain levels as do single classes. Go one level down or up, and the other character has the upper hand.