Those intervals are specified in terms of ratios. The absolute intervals are a direct function of the sending speed - the ratios remain fixed, the actual intervals change according to the notional sending speed.

The complication for detection is that not only do you need to be able to compensate for a drift in sending speed over time, but also you have to accommodate deviations from the ideal ratios of the various intervals which may occur on a letter by letter basis, and may also change depending on the context of individual letters.

Autospeed detection is actually essential for a practical decoder because the speed does change over time. A hint can help, but it's unlikely to be enough on its own.