They spend a lot of time and money on constructing a board which gives them some pseudo-objective means of sorting players and options. However, they still understand any board they put together is often not going to reflect the reality of how one specific player will do. Even ranking players as first round talent will only get you a coin flip for success although certain positions can do better.
So you spend a great deal of effort to grade players because that's the best you can do knowing the entire league often gets it completely wrong.
The point of all that is you have to balance long term needs with short term ones basing those decisions on rankings you know are on average often wrong. So taking a starting quality OL which you need now much more than a starting quality DL is a no brainer. Getting your panties in a wad over picking a lower ranked player is based on the illusion that your board is an actual reflection of talent and success in the NFL.
Oh, and BTW, you shouldn't complain about not taking a higher ranked player on the board then also complain the board means nothing because Dallas never gets it right anyway. One kinda undermines the other.