It all boils down to style.
Preterite "was" does actually imply that the situation was hypothetical, i.e. contrary to fact. Although it’s probably obligatory here ("is" would be questionable) by virtue of the matrix clause being in the preterite, it is regarded as a modal preterite (the kind with a modal rather than temporal meaning).
While it’s true that "was" is traditionally stigmatised in constructions like this, with many speakers feeling that "were" is stylistically preferable, I don’t think it’s semantically essential here, and hence it’s a free choice with style being the deciding factor.
Incidentally, I much prefer the term 'irrealis mood' to describe this "were". Subjunctive is a kind of construction, not mood, and the term 'past subjunctive' is quite ill-named.