These two verses you have posted are not linked. They are talking about two different things. The first is in surah Al Haqqah:
And lo! it is a warrant unto those who ward off (evil).(69:48)
This actually better links with the verse from surah Baqarah:
This is the Scripture whereof there is no doubt, a guidance unto those who ward off (evil). (2:2)
The word used in the two verses is Muttaqeen, sometime translated as righteous/god-fearing/those who ward off evil.
And the Quran is a book for the righteous, in it commands many righteous acts, for example it tell use to fight against oppressors, to command justice etc...
Your second verse from surah Adh-Dhariyat:
51:55 And remind, for indeed, the reminder benefits the believers.
We are insaan/human. The root word for insaan has the meaning the forgetful one. It is in our nature to forget. Sometime we forget our purpose of life and chase after dunya/worldly things. The Quran help us to remind us our true purpose. You will notice the next verse after this one you have posted is:
I created the jinn and humankind only that they might worship Me. (51:56)
This one verse sums up the purpose of existence.
But I do not say by this answers that a Mumineen/believer cannot be righteous/mutaqeen.
The reason this verse does not mention the unbelieves/kafir, is because the previous verse Allah is telling Muhammad (pbuh) to momentarily withdraw from the unbelievers:
So withdraw from them (O Muhammad), for thou art in no wise blameworthy, (51:54)
Hence don't preach the unbelievers now (at this moment) but still remind the believers. If you read it in context it makes sense.