No ... and yes.
What you say is absolutely correct of the most classical of classical Latin, and if you were translating English to Latin you should certainly use the present indicative with dum
But in later Latin dum
meaning while becomes assimilated to temporal cum
, and is commonly used to mean when or while, and so used with an imperfect subjunctive. Vide Woodcock (§ 221, n.ii):
Dum in this sense means practically the same as cum with the imperfect subjunctive in narrative. ... In colloquial Latin, in poetry, and in prose from Livy on, the construction of the two is apt to be confused, and dum is found in the imperfect subjunctive in contexts where there is no idea of "proviso" or "intention" to account for it.
Why choose that meaning here? Because I can't make sense of dum
here as meaning "provided that". Maybe in context I could. But the sense of the sentence seems to be "when", and I think it should be so translated. It's not just that translating dum
as "provided that" would change the meaning: it would change the meaning to something very odd indeed! (And the sense seems to have a Christian overtone, which would point towards later usage too.)