It appears that you may be misinterpreting the intent of the reference to "pride" as shown in the translation of the Igeres HaRamban.
Tehillim - Psalms - Chapter 93
יְהֹוָה מָלָךְ גֵּאוּת לָבֵשׁ לָבֵשׁ יְהֹוָה עֹז הִתְאַזָּר אַף
תִּכּוֹן תֵּבֵל בַּל תִּמּוֹט
The Lord has reigned; He has attired Himself with majesty; yea the Lord has attired Himself, He has girded Himself with might. The world also is established that it cannot be moved.
The word that you translate as pride is translated here as majesty. That is, since Hashem is the ruler, He is the one who "dons majesty". Thus one who attempts to exhibit pride or majesty is stating that he is the ruler and takes precedence. The analogy is to one who attempts to wear the robes (or crown) of the king. This could, in modern times, be like wearing judicial robes in a court room. Only the actual judge is allowed to wear judicial robes. It would be like the idiom of "donning judgement".
I think that this is the way the Ramban is expressing the idea in the Igeret. The translator used "pride" in the way that I used Judgement above or Majesty in the Chabad translation.