It isn't surprise at all. J-20's projected price is slightly higher than Su-27/J-11 at the time of program start. So J-35A has to be a lot cheapper to be worth the effort.
A good part of the rationale, but not completely.
Other big would be to increase production rates of LO fighters at multiple sites. And also allow multiple enterprises to accumulate experience in productionizing current gen platforms.
The most important reason, for the near to medium term, would actually to increase the numbers and availability of stealth platforms. If you have 140-180 airframes of 5th per year, it would be far faster to equip the bulk of PLAAF with latest fighters, than with J-20 alone.