I think this is a puritan statement of his, which to some degree rings very true but everything relative and depends on the desired result.
1) At lower volume most octals can be run in any octal socket amps, at least for some time. The differences there may be is attributed to the resistors of the grids and the transformers. Obviously the PT will have to take the filament current or it will blow.
2) You may argue you can design an amp around a specific type, even from a specific manufacturer on a) electrical operating points and b) tonally for a certain effect or that the frequency bands don't overbear. Then some Marshall JTM-45s w. KT66s did have OTs matched for EL34 output impedance and were thought of as the best sounding of its type.