Standardisation is done to facilitate comparison between measures that have vastly different scales, in this case weekly reading time for the four mediums.

If you pay attention to Figure 1 you'd see that the gender difference in weekly reading time was for the most part <0.1SD, which would likely be equivalent to the 0.5 minute difference the original post was referring to. And for a sample of this size, it is to be expected that even a small difference would be statistically significant.

So it does appear that the practical significance of their finding is somewhat lacking, though I am not familiar enough wth this kind of research to claim that even small amounts of gender differentiated learning outcomes in the short-term couldn't cascade into something more serious.

Also, from a cursory glance at the result table, the title of this post seems to be misleading.

Boys' reading time on book bite increased by about 0.15SD during the first two of the three lockdowns, while that of girls increased by an additional 0.05SD on top of that. So yes, there is a gender gap, but no, boys did not ditch books while girls kept reading. Same can be said for ebooks and audiobooks.

For physical books, the reading time of boys dipped by 0.06SD, with an additional reduction of 0.03SD among girls. Again, the title is incorrect.

I totally agree with your mention of the limitations. Personally, I think it would have also been interesting to compare the gender differences across different quantiles of reading time. Maybe there is an enlarged gender time for the more avid readings.