Seminarian give the homily?

[Digitonomy]

I’m confused! Surely a deacon can preach, and a seminarian who is a deacon can preach, therefore it is not true to say ‘Seminarians can’t preach’ quite so emphatically?

The most that can be said is that ‘Some seminarians can’t preach’ or, if you prefer, ‘Some seminarians can’.

No one really replied to your point - you are quite correct that seminarians can preach, but only if they are deacons. Seminarians cannot preach if they are laypersons, which is what benedictgal meant and what that paragraph of Redemptionis Sacramentum said. One should note that the paragraph is addressing only laypersons, thus clerics who happen to fall into these categories are not prohibited:

[66.] The prohibition of the admission of laypersons to preach within the Mass applies also to seminarians, students of theological disciplines, and those who have assumed the function of those known as “pastoral assistants”; nor is there to be any exception for any other kind of layperson, or group, or community, or association.