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Another point made in the same paragraph is that the Chief boasted being able to trim the boat so well at any other depth, that it literally 'hovered', either sinking or rising very slowly. Depth control at this trim being by slow speed. The speed is not mentioned but all indicators point to less than 2 Knots.
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Dynamic depth keeping at slow speed is normal.
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The author makes distinct reference to the chief of the boat being able to 'Hang by the Periscope' - which essentially is hovering at periscope depth with the scope extended with engines off, IF the sea conditions were suitable.
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Neutral buoyancy is more of a theoretical condition than an actual one, in practice you are always going to have either positive or negative bouyancy, if only by a small margin. I suppose that for an u-boat hovering at periscope depth for a significant lenght of time would have indeed been possible in a calm enough sea, but it would have probably required a skilled operator and very frequent trimming and compensation (with all that these entail). Italian WW2 submarines had been equipped with an automatic apparatus for static hovering at shallow depths in the 1930s, but it was eventually removed during wartime refits as such capability was not worth the trade offs (noise etc.).
My two cents.
EDIT
Rereading the actual passage now I get it at last.They are referring to the use of the periscopes for buoyancy control. From what I read this trick was indeed used, but will only work for a limited amount of time, eventually you have to resort to pumping.