Prog can be anything really, just generally longer songs, maybe with a developing theme across all songs. Maybe less in the way of instant punchy choruses.
If you think about it, what could be more "prog" than a concept album? And what was Secret of the Runes? - a concept album about Norse mythology.
Clavicula and the like also fit the bill, but there is such a wide range of things that could be called prog that it really is impossible to tell. I even read a (bad) review of Lemuria where Therion were already described as a prog band anyway!!
I think that this statement that Chris made should not alarm anyone who is a fan of the band. To me it sounds like he really wants to try and push the boundaries as to what he can achieve. Big songs, Big arrangements, Big concepts...
I think we are in for a Masterpiece (yet again)!!