What is indirect expansion? What does ${!var*} mean?

There appears to be an exception when the given "indirection" ends in a *, as it does here. In this case, it gives all variable names that start with the part you specified (N here). Bash can do that because it tracks variables and knows which ones exist.

True indirection is this:
Say I have a variable $VARIABLE set to 42, and I have another variable $NAME set to VARIABLE. ${!NAME} will give me 42. You use the value of one variable to tell you the name of another:

$ NAME="VARIABLE"
$ VARIABLE=42
$ echo ${!NAME}
42

If you read the bash man page, it basically confirms what you have stated:

If the first character of parameter is an exclamation point (!), a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion.

However, reading on from there:

The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below.

${!prefix*} Names matching prefix. Expands to the names of variables whose names begin with prefix, separated by the first character of the IFS special variable.

In other words, your particular example ${!N*} is an exception to the rule you quoted. It does, however, work as advertised in the expected cases, such as:

$ export xyzzy=plugh ; export plugh=cave

$ echo ${xyzzy}  # normal, xyzzy to plugh
plugh

$ echo ${!xyzzy} # indirection, xyzzy to plugh to cave
cave

Yes, it searches for all possible expansions of variables after the !. If you had done:

echo ${!NP*}

you would get only NPX_PLUGIN_PATH.

Consider the following example:

:~> export myVar="hi"
:~> echo ${!my*}
    myVar
:~> export ${!my*}="bye"
:~> echo $myVar
    bye