XML DML modify practice question in 70-431

I am currently studying for the 70-431 exam, and battling with understanding XML

In one of the exercises, they want you to modify a record that looks like this:

<logRecord machine="server2" timestamp="2000-01-11T12:13:14Z">
  <information flag="failure">
    <message>Original message was here</message>
  </information>
  <error number="18763">
    <message>Application can not start</message>
    <module>AppLoader</module>
  </error>
  <post eventType="appStart" />
</logRecord>

To this:

<logRecord machine="server2" timestamp="2000-01-11T12:13:14Z">
  <information flag="failure">
    <message>Not enough memory</message>
  </information>
  <error number="18763">
    <message>Application can not start</message>
    <module>AppLoader</module>
  </error>
  <post eventType="appStart" />
</logRecord>

Not wanting to experiment with the underlying data, I wrote the following script:
declare @x xml

select @x = LogRecord from UniversalLog where ApplicationName = 'HoursReport'
set @x.modify('
      replace value of (logRecord/information/message)[1]
      with "Not enough memory"')

select @x

This fails with the following message:
XQuery [modify()]: The target of 'replace value of' must be a non-metadata attribute or an element with simple typed content, found 'element(message,xdt:untyped) ?'

Yet in the completed answer they give the following, which does work:
UPDATE UniversalLog
SET LogRecord.modify('
      replace value of (logRecord/information/message)[1]
      with "Not enough memory"')
WHERE ApplicationName = 'HoursReport'

This is the same modify statement, and it works OK.

Why would it not work with my experimental way?

Thanks!

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