iPhone devices impose limitations on the amount of time the shutter is open - it varies from about 0.5s in an iPhone 5s to 1s in the iPhone X family.
Camera+ 2's slow shutter mode works like this: it takes a sequence of frames and combines them into a single photo. That works great for techniques like blurring moving parts, and it can even be used in daylight because we attenuate the received light to avoid overexposure. However, it is not the same as having your shutter open for a long time.
Unlike what happens in DSLRs and other cameras, there is no way in current devices to force the shutter to remain open. This is a system limitation that we can't circumvent. Presumably all long exposure apps out there use similar techniques.
As soon as new hardware and/or system APIs allow direct control over the shutter, we'd be delighted to create a shooting mode that takes advantage of that feature.