The prohibition to attract subcontractors and co-executors corresponds to the Civil Code of the Russian Federation. The law allows to include in the contract a condition on personal performance by the contractor of works, rendering of services.
The said ban violates the law only if it is proved that with its help the customer wants to win the victory of a particular bidder.
Document: Review of the judicial practice of the Supreme Court of Russian Federation No. 5 (2017), approved by the Presidium of the RF Supreme Court on December 27, 2017 (http://www.vsrf.ru/documents/practice/26303/)