HI @mrplum,
when you add the CSS there, it will be mixed with other CSS rules on the front end, so the @import statement will not actually be at the top of the decalration. If you take a look at the generated CSS, you will see that it’s inside one stylesheet.
Unless, I think, if you use “external styles”, then I think it should be rendered on it’s own.
I think enqueuing it via PHP using a code snippet or child theme is safest though, as you have full control over it… 
Best regards,
Matej