The +5 comes from assuming a 50% chance to hit. That means that you miss on 0.5*0.5=0.25 (or 25% of the time) and you therefor hit 75% of the time. This is equivalent to +5.
Now, you would be correct in pointing out that this is the maximum benefit possible from advantage. It declines as you both raise and lower your chance to hit. It does overestimate the value of advantage a bit.
However, it is almost certainly a better estimate than assuming an even distribution of hit chances. In a well balanced game you will not see very many situations where you hit on a 2, or very many situations where you only hit on a 19 or 20. Given the actual to-hit chances that you will encounter in BG3 the value is probably between +4 and +5.
In my gaming experience, the base chances to hit were generally way above 50% (at this moment I can't pinpoint a number, though). It is very well possible that Larian adopts a "missing is boring" stance, so we get something like a 80% base hit chance on average. In that case, advantage would only translate to +-3. I think assuming an even distribution is as accurate as we can get until the numbers are tweaked.
As I said in my earlier post, +5 is an exaggeration of the benefit of advantage. I still do not agree with you that a flat distribution is the best estimate, since you are allowing the extreme cases to drive the average too low in this case. If you truncate the distribution to eliminate very low (only 19 and 20 hit) and very high (anything 3 and above hits) you already get to a +4.067 (calculated via your Wolfram link).
I also think that you are underestimating the importance of small boni in 5E. This edition of D&D operates under a scheme called "bounded accuracy" which makes bonuses to hit very hard to come by. Stat bonus will max out at +5 (unless you are a level 20 barbarian, which is very much an edge case) and proficiency will max out at +6. If Larian follows the treasures in the DMG then the maximum magic item bonus will be +3...
Under this regime ANY bonus is impactful, even a lowly +1. This is not pathfinder where you can stack bucketfulls of feats, items, and stats to achieve a +75 to hit if you min/max up the wazoo.
