ekaggata is not a 1st jhana factor
secluded from sensuality, secluded from unskillful qualities, enters & remains in the first jhana: rapture & pleasure born of seclusion, accompanied by directed thought & evaluation.
for instance AN 9.36
and incidentally this sutta seems to suggest that the 1st jhana suffices for arahantship
I tell you, the ending of the mental fermentations depends on the first jhana.
and after all the jhana of his childhood experience the Buddha recalled under the Bodhi tree was the 1st one, this of course doesn’t mean that in the process of awakening he didn’t progress up to the 4th jhana but in the light of this sutta it would have been unnecessary
from this sutta it’s also to be inferred that unless they attain arahantship a jhana practitioner is destined for non-return
Staying right there, he reaches the ending of the mental fermentations. Or, if not, then—through this very dhamma-passion, this very dhamma-delight, and from the total wasting away of the first five of the fetters—he is due to be reborn [in the Pure Abodes], there to be totally unbound, never again to return from that world.
if the noble level immediately associated with jhana is non-returner, it means, just as Ven Analayo concluded, that all preceding noble levels don’t necessitate jhana