even tho urgots ult shows "9999" damage, thanks to freaks like vandiril we know its higher. (watch his 8 hour long custom game where he urgot ulted a sion with over 100.000 hp, it still executed him)
so yeah deaths dance + urgot ult = full hp.
edit: doesnt matter that it is a dummy. urgot ult always does 999.999 true dmg no matter the target (it just shows 9999) and you heal for 15% of damage DEALT