Yes, that explanation is reasonable. Camera lens mounts are commonly made of steel or stainless steel, and steel can rust if exposed to moisture. Salt water or even salty sea air can speed this up significantly.

Not all metals “rust” in the orange-brown sense: aluminum oxidizes differently, while steel is much more prone to visible rust. Based on the community answers, the Canon 70D mount is believed to be steel, so slight rust staining is plausible if it has been exposed to water or humid/salty conditions.

If the rust is only superficial, it may just be staining. But if it starts to interfere with mounting lenses smoothly, the mount may need service or replacement. To prevent further corrosion, keep the camera dry, wipe off moisture promptly, and be especially careful after shooting in rain or near the sea.