Is my interpretation of `sed -i.bak '/^x /d' "$SOME_FILE"` correct?

Yes, your understanding is correct. From your format, I assume you are using GNU sed (other implementations might need a space between the -i and the .bak and some might not support -i at all). Its -i works as follows (from info sed):

-i[SUFFIX]
--in-place[=SUFFIX]
     This option specifies that files are to be edited in-place.  GNU
     `sed' does this by creating a temporary file and sending output to
     this file rather than to the standard output.(1).

 This option implies `-s'.

 When the end of the file is reached, the temporary file is renamed
 to the output file's original name.  The extension, if supplied,
 is used to modify the name of the old file before renaming the
 temporary file, thereby making a backup copy(2)).

 This rule is followed: if the extension doesn't contain a `*',
 then it is appended to the end of the current filename as a
 suffix; if the extension does contain one or more `*' characters,
 then _each_ asterisk is replaced with the current filename.  This
 allows you to add a prefix to the backup file, instead of (or in
 addition to) a suffix, or even to place backup copies of the
 original files into another directory (provided the directory
 already exists).

 If no extension is supplied, the original file is overwritten
 without making a backup.

The d command deletes any line on which the previous expression was successful. Strictly speaking, it deletes the "pattern space", but in simple sed scripts that is the line.