'In the day' meaning 'in one day'

The use of 'in the day' in this paragraph puzzles me:

There were more than 800,000 horses on the Eastern Front in 1941. Mr Baxter said that, in the day, around 8,000 were being killed by enemy fire, the freezing temperatures or sheer over-exertion.

Does this mean 'in a day', and if so, is such usage correct?

Source: https://www.dailymail.co.uk/news/article-9516507/Rare-photos-Operation-Barbarossa-reveal-horrors-faced-Hitlers-troops.html

update: I believe that the intended meaning must have been either "in a day" or "around the time the picture was taken" (the paragraph is a caption for a picture (of a dying horse surrounded by soldiers)).

Solution 1:

As you suggested, I also think he means to say that "in the day" (in one day/across the duration of the day), about 8000 horses died.

At least contextually, this makes the most sense to me. They had 800000 horses and about 1% died each day.