It’s absolutely true. As noted above, your assumptions are, still, incorrect. There is one rendering pipeline, it’s not being divided between the cards but handled by the primary GPU and therefore limited by it’s bus width, vram and raw power. The specs of the other cards are essentially irrelevant.
As Crunchmeister71 notes, the other GPUs are just acting as display adaptors.
Last post here. If you’re happy, great.