Sterile cockpit does not mean you can’t talk.
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engaging in non-essential conversations within the cockpit
If I say ‘nice sunrise but now I’ve got the sun in my eyes can you adjust my sun shade’ is not a breach of sterile cockpit rules.
Neither is giving yourself progressive taxi and requesting confirmation from other crew members in the cockpit including jump seaters.
That is all pertinent to the specific phase.
So they could have been talking since pushback and as long as it was all relevant it’s not a breach of the rule.
Sterile does not equal silent.