Spells only do what they say they do.
The "insight" into the target's defenses is only the insight you need to gain a momentary advantage. The spell does not say that you can know the monster's resistances, HP, or anything else, especially because such insights are decidedly not "brief". Instead, the kind of insight you get is as @MarcusYoder described in a comment:
You cast this on an orc with a sword, you might get the a gut feeling that, "He will step too far out of his stance on his next swing and be vulnerable." ... This insight wouldn't be applicable after next round.
The (settled) debate about fluff or non-fluff in 5e spells does not apply here: either way, it is still only a brief flash of insight and thus precludes any more detailed knowledge.
Comparison with a class feature
Whenever the system does give you some insight into a creature's mechanical stats, it is specific about what you learn and how you learn it. The Battlemaster's "Know Your Enemy" states:
Starting at 7th level, if you spend at least 1 minute observing or interacting with another creature outside combat, you can learn certain information about its capabilities compared to your own. The DM tells you if the creature is your equal, superior, or inferior in regard to two of the following characteristics of your choice...
If True Strike was to give you such information, it would say so explicitly.