Subway is perfectly willing to admit that most of their cold cuts are turkey-based, so it seems extra unlikely that they'd go to the trouble of creating an ersatz tuna and then hiding that fact. Especially considering how cheap tuna is, comparatively. The way you cheap out on tuna salad is adding more mayo and thickeners and other fillers, not by inventing some kind of psuedofish.
https://www.seafoodsource.com/news/foodservice-retail/subway...