The FIA strongly prefer manufacturer backing of any new entrant so this is believed to be why they teamed up with General Motors. GM evidently wants to boost Cadillac's global image so that is why they decided to run their part of the partnership under the Cadillac Racing division and brand the team Andretti-Cadillac.
Speculation is that Honda will provide the initial powerplants to get the team on the grid (since Andretti has a strong relationship with Honda Performance Development), but the goal is to make the team have more "American content" so I would presume GM would be willing to build a dedicated powerplant based on the 2026+ design.
I could see that I suppose. I didn't factor in Andretti's Strong relationship with Honda in Indy Car. Given the financial constraints Honda is under currently it would be remarkable to see them providing that level of support to 2 F1 Teams thru 2025.
RBRE is still heavily operating with Honda Engineering "Consultants." Do they even have the people? Who decides which Engineers go where?
I just don't see Cadillac producing a PU in 2026.