"Actually, breath is [an] uncountable noun". No, it isn't. The noun 'breath' can be uncountable or countable: "Suck some peppermints. It'll make your breath smell nice and minty". = uncountable. "Relax. Take 10 deep breaths" = countable It's actually very normal to use 'breath' as a countable noun, meaning one single inhalation of air. For example, we usually use the countable meaning in everyday expressions such as 'every breath you take' or 'his last breath', or figuratively 'a breath of fresh air' or 'not a breath of wind'.

In fact, in this context, 'breath' is countable.

a breath = one breath. It's a different thing. 'breath' in general is uncountable, but this is one breath, like 'breathe in, breathe out'