I think that this is related to the following: http://php.net/manual/en/migration70.incompatible.php
Essentially adjusting the indirect variables and the problem would be resolved.
My suggestion would be to change the following in the wordpress-importer.php file in line 798 where it reads:
$$meta['key'] = $meta['value'];
to be inline with PHP 7 and backward compatible to the following:
${$meta['key']} = $meta['value'];
