The paper actually deals with $\pi/4$-QPSK, not with plain DQPSK. In $\pi/4$-QPSK, the symbols are always rotated by a minimum of $\pm \pi/4$. Rappaport's Wireless Communications textbook has this table:
11 \rightarrow & \, \pi/4 \\
01 \rightarrow & \, 3\pi/4 \\
00 \rightarrow & \, -3\pi/4 \\
10 \rightarrow & \, -\pi/4
The diagram from the textbook in the opening post appears to have some mistakes. For this IFFT method, the box that says 'QAM modulator' should just be a complex number generator (such as a digital QAM generator that produces a complex number from 'x' binary bits at a time - that is meant to represent a vector on a QAM grid).
And there should be two D/A ...