It is explained lower in the article.i.e in the reciprocity theorem according to which
"If Antenna A1 is a transmitting antenna and antenna A2 is the receiving one;
the ratio of transmitted to receive Power Pt/Pr will not change if
A1 becomes the receiving antenna and A2 becomes the transmitting one"
Here P1-2 is the receive power when A1 is transmit and A2 is receive antenna and
P2-1 is the receive power when A2 is transmit and A1 is receive antenna
P1 = transmit power by antenna A1
P2 = transmit Power by antenna A2
then according to reciprocity theorem:
P1/P1-2 (Pt/Pr if A1 transmits) = P2/P2-1 (Pt/Pr if A2 transmits)
Now imposing condition P1 = P2;
results in P1-2 = P2-1