As demonstrated in an older article cited in the parent article, at the same pace of 6 breaths/min it matters a lot which is the ratio between the exhalation time and the inhalation time.

In the experiments, slow inhalation with fast exhalation was never helpful, equal inhalation and exhalation was helpful only in certain circumstances and fast inhalation with slow exhalation (i.e. 2-second inhalation followed by 8-second exhalation) was always efficient in stimulating the parasympathetic nervous system and inhibiting the sympathetic nervous system.

The results from TFA are specifically for fast inhalation and slow exhalation, not for slow breathing in general.

The negative results from the article linked by you are perfectly consistent with the other results, which showed that equal inhalation and exhalation was useful only in certain circumstances, which were not tested in the article linked by you.

In general, slow breathing by fast inhalation and slow exhalation (or any other kind of slow breathing) does not have effects when you are already relaxed and having nothing to worry about, but only when you are stressed, either by anticipating that something bad will happen or while something bad is actually happening.

I've done coherent breathing before bed, 20 minutes. Improves my HRV by 20-30%