EditPlus (as of v2.12) does not support an option to allow proper syntax highlighting of nested comments. Ostensibly this is because many programming languages do not support parsing of nested comments.
The following is an example of what a nested comment would look like in HTML:
<!-- This is a comment <!-- This is another comment --> End of the first comment -->
However, HTML does not support nested comments, thus the code above would be rendered in a web browser as:
End of the first comment -->