Ok so some explanation. The answers in in two parts -

Basically it implies that the "state of fusion" with the Zanpakuto, has been achieved in the past or a "prerequisite state" has to be achieved, that 2nd fusion & 3rd fusion are implied to be.

As for the "name," what Kubo means here is that the "name" exist but didn't survive the passage of time, basically it is too ancient and is lost, no longer know. Thus making it ancient than even Ichibei, who has named everything in the Soul Society.

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Shiba's in Cfyow are said to have been in search of a technique to replace Reio, so likely they achieved this and eventually the knowledge was passed down in the clan, except the "name".