"Modern dev tools" doesn't solve everything. Bioware already had a well-working engine for BG2, and I assume they were able to carry over certain assets and other aspects from the predecessor.
Art asset creation, writing, creating interesting gameplay, all these things still take a lot of time, and dev tools make very little difference there.
They've given themselves plenty of time and well exceeded their original asking price. The art asset creation is significantly easier with the engine they're using and the writing has probably already been taking place for years.
Obsidian is going to be working with an engine that will allow them a very speedy development process that will not cost them anything (relatively speaking).
You could go out and buy Unity right now for pennies on the dollar compared to other, less development friendly engines.