The short open tag <? ?> is different from <?= ?> as I explained above and should not be used.
Kirby requires PHP 5.4+, so the short echo tag may safely be used. PHP 5.4 has reached its end of life, anyway: http://php.net/supported-versions.php
You should be using PHP 5.6+ these days.
See this discussion as well: To PHP Short Tag or not to PHP Short Tag?