calling it illegal with this argument is incorrect.
I tend to agree with Andrew Hammel who thinks the question is largely irrelevant given the complete collapse of asylum law in numerous EU countries which multiple European leaders knew was a danger and did nothing to prevent....
"But in terms of relative illegality, it's like charging someone who committed millions in tax fraud with failing to use adequate postage on his fraudulent tax returns"*
The important point is that European asylum law is completely broken and no improvement is in sight because no one who can wants to deal with the mess....
*here: hammeltranslations.com/2019/05/09/more-on-the-2015-refugee-crisis-and-the-rule-of-law-a-response-to-steinbeis/