This may be a unique feature. Certainly many TLTP projects have been characterised by teams of authors who develop material and then go around wondering why so few other people want to use their wonderful material, even though they themselves would never use anyone else's. This was chosen because it is a typical unit for planning courses. Small units of exchange are very important. Firstly, even apart from the powerful forces of idiosyncratic preference in a deliverer, different situations in different HEIs and courses require different materials.
Small units increase the chances of being able to use some of what is on offer from elsewhere.
Secondly, it is much easier in practice to introduce a few ATOMs on an existing course than to redesign the whole course. Thirdly, most deliverers will want to try out only one or two ATOMs to see if it is going to work out before committing to greater use. Exchanging material matches this distribution of expertise, while avoiding difficult accounting issues that would be raised if the effort became seriously asymmetrical. After taking an ATOM from a remote expert once or twice, I will probably now feel expert enough to deliver it myself without further external support. If that is the pattern that emerges, then much reciprocal collaborative teaching will be viewed as professional development rather than as permenent dependence on other HEIs.