The reason for this in the code goes back a long way. I think that there are two reasons: consistent functionality across tables and plots, and also being able to have space separators with a “fake” space. What I mean is that if you copy a number from a MultiQC table with a space separator, it’s copied without a space. This is because it’s actually a HTML <span> element.
It could be nice to revisit this code in the future and see if we can refactor it to work in a better way.
A note for future reader (probably me…), if you do change the format string, you can only use f, and if you want the thousandsSep_format to be honored, you still need to include the comma in your format string.