Yes, March 2026 is when the first announcement from the OEM will be which is when people will find out which OEM it is. A fair bit into 2027 is when the devices supporting GrapheneOS will be launched. It was theoretically possible for it to happen for 2026 but not very realistic and the hardware wasn't quite there yet.
In my opinion (which is worth what it is worth), 2027 is perfectly fine (my Pixel should live a lot longer than that anyway). What I find huge is that it is concretely happening! It will show recognition for GrapheneOS, and it will be great to have an alternative to the Google Pixels.
Also, people looking at GrapheneOS want technical excellence, so better not rush it :-).