"And somewhat spoiling what was for me an excellent recording otherwise"
It doesn't spoil it for me at all. On the contrary, it addresses what I perceive to be, quite frankly, a slight 'shortcoming' in Mahler's orchestration at that particular spot. Since no extra brass are added on the last three chords, there's nothing to make those final chords stand out from the previous measures. The bass drum gives them a bit more 'final' impetus, in feeling.
The same sort of problem exists at the very end of M2 and M8. If the second timpanist doesn't make a massive crescendo on the final chord of M2, that final chord sounds like it's just sitting there doing nothing - static, in other words. Pretty much the same thing can happen at the end of M8, which is why some conductors make a big crescendo with the bass drum - or even add a gong roll (Segerstam).
