I feel that tptacek's point is that excessive cipher agility in TLS precluded adoption of AEAD (and "unconventional" modes in general), because it was built on

I feel that tptacek's point is that excessive cipher agility in TLS precluded adoption of AEAD (and "unconventional" modes in general), because it was built on assumption that the interchangeable primitive is the block cipher itself (ie. keyed primitive that transforms n bit input to n bit output) and not the combination of cipher+mode (which transform arbitrary length input into some, preferrably longer, output). You cannot plug AEAD into framework that expects block cipher and MAC. (Due to how all this is really implemented, this in fact was not technical problem, but problem of "design culture" or something like that)