If that's what Richter is reported as saying, then maybe his comments have been misrepresented in translation. I'm sure that Richter would have been aware of performances by Artur Schnabel, Wilhelm Kempff and others which had been given long before Richter started playing Schubert's sonatas in concert.
I remember hearing this March 1979 recital broadcast by the BBC soon after Richter gave it; one of the sonatas has been omitted here as there wasn't space for the whole recital on the CD. I think you sum up the qualities of this recording succinctly. There are many fine recordings of Schubert's sonatas, but Richter's is special, precisely for the reasons which you give.
The coughing by the London audience is a disgrace though. Thankfully there are other Richter Schubert recordings in which we are spared the audience's insensitive contributions - the Philips/Decca account of the Sonata in G, D894, also from a London concert, is a marvel - but this BBC disc is certainly worth having.