First, While your archy is technically correct, a monolithic foundation generally refers to a slab on grade where the footers are paart of the slab, ppoured at the same time, making it monolithic - all one. You cannot do walls that way because you need the footer poured first to be able to form for the walls.

A re-inforced concrete wall is always stronger than a wall built of CMUs, even with the rebar and solid fill your contractor recommends. The question is whether that extra strength is needed. Are your soils unstable, clay, or wet? Do you have heavy freezing? Is your house extra large and heavy or designed with a lot of jigs, jogs, and angles?

In many parts of the country, laying a foundation wall from individual CMUs is the standard practice because it is compatable with the local soils and climate. In other places, Re-inforced concrete is the norm.

It could be that your archy has a background in another place, or it could be that your contractor is limited in his vision and experience.

Ask around locally. Ask the contractor whether he can show you work he has done similarly ten years ago. Ask the archy why he prefers the concrete.

Finally, compare the costs for each. Then decide if the price is worth it to you. After all, it's your dinero.

Your architect is correct in preferring concrete walls to masonry block walls. Even if the block walls are filled with concrete and rebar the solid concrete walls are much stronger. In a full basement the walls will likely be holding back a great deal of earth. This earth has the potential to move over time with freeze-thaw cycles and wet-dry seasons. Block will have a much higher potential for cracking and is harder to keep watertight. Block is fine for a crawl space but I would also recommend using reinforced concrete for the walls in a full basement.

Glenn