Senna is a support who is partly an ADC. While she can be played as an ADC, that is not her intended role, and they will nerf her to stay a support if her ADC side gets too strong.
There's also this: https://i.imgur.com/5xsaoAn.jpg[]
And this: https://i.imgur.com/vFt6NAu.png[]
All that being said, Aphelios is likely the marksman after Senna, and it looks like they will in fact be a Lunari. As for why they are a marksman, I think its more to get a marksman who is actually intended to be played in a damage role again. Its been well over a year since we got Kai'Sa, after all, and Targon is lacking a marksman champion. Thus, Targon marksman right after Senna.
I wish they had put the Ionian Juggernaut first, just to space things out a little, but if they want to tie in Aphelios with Diana's rework, then I can see why they're doing them first.