Interesting question:
Answer: By Bp. Fellay's Six Conditions" he has already "... approved and determined" to make a Practical agreement in lieu of a Doctrinal Agreement.
Therefore, he has consented, in writing to: "A danger to the Faith", as taught by St. Thomas Aquinas.
Therefore, we... the SSPX-Resistance, "the inferiors", have a duty to correct the superior (Msgr. Fellay), even in public, because the Practical agreement is a danger to the Faith.
Reference: SSPX-SO