Date end.
There are many posts, not just one.

Okay, so there's only 1 Posting which is older than 14 days and only this one would be deleted if you would add the delete code at the bottom.

You may experiment with changing '2 WEEK' to '10 DAY', '5 DAY', '1 DAY' or whatever you like
and you'll see: the smaller the number of days, the more records you'll get.

So the code is working as expected, and I don't see what your problem is?

php /home/elittagh/public_html/cron/posts.php
POSTINGS <pre>1</pre>

Fine, so let's revert the code back to the orginal condition to find only older records.
Remove the three double slashes like

 <?php
    define('OSSN_ALLOW_SYSTEM_START', true);
    require_once ('/home/elittagh/public_html/system/start.php');
    $wall = new OssnWall;
        $listUser = $wall->GetPosts(array(
            'type' => 'user',
            'page_limit' => false,
            'wheres' => array(
                "(FROM_UNIXTIME(o.time_created) <= DATE_SUB(NOW(), INTERVAL 2 WEEK))"
            )
        ));
    echo "POSTINGS " . ossn_dump(count($listUser));

and execute the command again.
How many postings do get this time?

$ php /home/elittagh/public_html/cron/posts.php
ALL POSTINGS <pre>31</pre>

You can simply change that last line to

echo "ALL POSTINGS " . ossn_dump(count($listUser));

to see the total number of postings

Okay, so it's basically working.
The interesting part is: how many OssnWall Objects?
I see only two:
[0] => OssnWall Object
and
[1] => OssnWall Object

and both are fairly new

Lots of text, can't be uploaded here.

$ php /home/elittagh/public_html/cron/posts.php
POSTINGS <pre>Array
(
[0] => OssnWall Object
(
[data] => stdClass Object
(
)

        [guid] => 1239
        [time_created] => 1631263197
        [owner_guid] => 3
        [description] => {"post":"Hivatalban,adatfelvételkor....\r\n-Vezetékneve?\r\n-Kábel!\r\n-Nem úgy!Hogy hívják?\r\n-Jaa..."}
        [title] =>
        [type] => user
        [subtype] => wall
        [item_type] =>
        [item_guid] =>
        [poster_guid] => 3
        [access] => 2
        [time_updated] => 0
        [linkPreview] =>
    )

[1] => OssnWall Object
    (
        [data] => stdClass Object
            (
            )

        [guid] => 1238
        [time_created] => 1631210960
        [owner_guid] => 2
        [description] => {"post":"Hamarosan indul a nyereményjáték, aki a legtöbb igazolt személyt tudja meghívni az Elittagságba, azt jelentős nyeremény várja majd!"}
        [title] =>
        [type] => user
        [subtype] => wall
        [item_type] =>
        [item_guid] =>
        [poster_guid] => 2
        [access] => 2
        [time_updated] => 0
        [postbackground_type] => pbg3
        [linkPreview] =>
    )

Yup, so let's debug:
Put this code in another file named posts.php in the same directory

<?php
define('OSSN_ALLOW_SYSTEM_START', true);
require_once ('/home/elittagh/public_html/system/start.php');
$wall = new OssnWall;
    $listUser = $wall->GetPosts(array(
        'type' => 'user',
        'page_limit' => false,
//        'wheres' => array(
//            "(FROM_UNIXTIME(o.time_created) <= DATE_SUB(NOW(), INTERVAL 2 WEEK))"
//        )
    ));
echo "POSTINGS " . ossn_dump($listUser);

then execute it like

php /home/elittagh/public_html/cron/posts.php

and tell what the output looks like

It only deletes it if I put the code in index.php.