Given only the isolated quotation, there is another interpretation that complements @Richard's. By "merely lyrical", I understand Cohrs to be saying that themes can be both lyrical (i.e., melodic) and ... have other additional qualities that serve as compositional material. In the Bruckner case, the themes are "merely" (i.e., "only") lyrical.
What can a theme be in addition to, or instead of, "lyrical"?
@Richard's invocation of Beethoven 5 is outstanding both for its illustration of the lyrical/aggressive dichotomy, but also because the first theme is not lyrical/melodic; rather, the first theme is rhythmic.
The famous opening gesture of Beethoven 5 comprises and eighth rest, three eighth-notes, and a longer note. A look at the first phrase illustrates the point: it's constructed almost entirely from the same four-note rhythmic figure.
Again looking to the first theme of Beethoven's fifth, it is very much based around thirds. The opening gesture, of course, features two descending thirds.
K: C minor
z GGG | !fermata!E4 | z FFF | !fermata!D4 |
And thirds also appear in ascending and descending scalar patterns. For example, Violins I and II in mm. 14–15:
K: C minor
%%score V1 V2
[V:V1 clef=treble] z ggf | e4 | x4 |
[V:V2 clef=treble] z4 | z EEF | G4 |
Much serial music is also based heavily around intervallic material, tone rows being understood as series of intervals (or interval classes) rather than series of pitches.
Moving away from Beethoven, Chopin's Etudes are built primarily around particular playing techniques. His Op. 10, No. 1, for example, is built entirely of arpeggios. Op. 25, No. 6 is all about double thirds.