The development of the Plone REST API and Volto is a joined effort by Plone and Guillotina developers (and many others). Volto runs on both Plone and Guillotina.
It is hard to differentiate between Plone and Guillotina sprints, topics and tasks to be honest. Just take the Toulouse sprint as an example. Ramon, Nathan, and Victor will go to the Toulouse and the Beethoven sprint (one week after Toulouse) and they will basically work on the same topics. Albert will be in Toulouse and he will work on the Volto UX, which is one of the most important things we need to do right now in Volto. Nathan and Ramon (and others) will for sure work on further enhancing the API design of Plone REST API. Ramon was one of the first people I discussed the REST API design with (he basically wrote plone.rest alone tbh) and Guillotina implemented some of the API endpoints before Plone REST API did. Eric was one of the first people that started to work with Plone REST API and his fantastic work on Angular SDK helped shaping Plone REST API.
I understand that it might look like the Plone 6 efforts (plone.restapi + Volto) and Guillotina are seperate things. There are not. The Toulouse and the Beethoven sprint will be two extremely important efforts this year to push Plone 6 further.
We (as the framework team) understand that we did not do a very good job in the past of explaining our strategic sprint decisions. We already discussed how we make the decision process more transparent and we will try to do a better job. I hope Eric's and my comments made things more clear to you.