Hi @Heku_,
This info in the screenshot does mean that the mirror page for the delivery preview is available for viewing it (the preview itself) in the web browser.
The availability of mirror page here informs us whether the link to mirror page has been included somewhere in the delivery content or not. In case not, the message here will display "unavailable".
There is one more case where this will always show "available", when the delivery properties > validity tab > mirror page management is set to force generation of mirror page even if the mirror page link is not included in delivery content.
BR,
Ishan