I've been wondering for a while about this matter: why are there arhats in Sukhavati?
There are probably many reasons for this, but here's my 2 cents...
The Pure Land tradition already has a lot in common with the Pure Abodes of the Sravakayana and this statement further unifies the two. From the view of all forms of Nirvana/Enlightenment being the same, then it's not such a huge deal, as the life span in the Pure Lands/Pure Abodes is very long and a being Enlightened there would have ample time to help sentient beings. If you take it from the Ekayana point of view, the Pure Land equates to nonretrogression, that Sravakas are also on the path even if they have to generate bodhicitta later, and that either way they will be progressing. If you look at later sutras like Medicine Buddha, there are statements to the effect that once Sravakas arrive in the Pure Land, they are then encouraged to follow the Bodhisattvayana. Not sure if I buy into any of the polemics or triumphalism. A lot of this has to do with how you define Buddhahood and the requirements therein.