I bolded a different part of your quote. If you can be seen when you make an attack you are no longer "unseen" and therefore no longer "hidden"
There is no need to see how much time it takes to raise a shield, as soon as you poke your head out you are not
unseen".
While I am at it - you have disadvantage on enemies you can't see, so if your arguement is that sticking your head out and flinging a dagger is not enough time for an enemy to "see" you, then it is logically not enough time for you to "see" your enemy either. So you have disadvantage plus advantage. Now if you can see your enemy from your hidden position that is a different story, but if you are behind a wall or tree or something like that you can't "see" him while back there, your enemy is not "hidden" because he did not take the "hide" action but he is "unseen".