Is pain in the lower back with leg pain at the same time necessarily sciatica?

Could be. There are many possible reasons for this combination, sciatica would be high on my list, but it is not all inclusive.
No. No, many structures from the low back can produce pain in the leg. This includes the discs, the facet joints as well the covering of the nerve sac called the dura. In fact i see more pain from some of these other structures than true sciatica. The quality of the pain from sciatica is also different.