When the current administration cites national interests and America First, it is more of a keyword phrase to excite the base supporters rather than the result of any thoughtful analysis. In many cases where it superficially seems to benefit the U.S., such as import tariffs, the longer-term consequences are not so positive, such as shifting the financial burdens to other industries and degrading trade relationships with other countries. The arms deal with Saudi Arabia isn’t that critical in the grand view of our economy. There is some interest in a relationship with Saudi Arabia to preserve some stability in the Middle East and because they also have an interest in keeping Iran in check, which syncs with our affection for Israel. Secondly, the actual value of the arms deal has been grossly exaggerated by Trump and the benefits to the U.S., and even how real it is. Not only that, but we have a trade deficit with Saudi Arabia but Trump has never criticized that, as he has done the trade deficit with China, for example.
I suppose that any administration wants to spin the data to look good, although this administration has made its entire stance based on national interest, so constantly comes back to that to justify every boneheaded decision. The irony is that many decisions that are made in the name of national interest further isolate us from international partners, which is going to have a deleterious effect in the long run.
TL;DR: When the administration says “national interest” it is often cutting off its nose to spite its face for the sake of short-term political capital.