The reason your allies don't provide true cover is covered by the exceptions based rules formulation.
The definition of cover is "The target is around a corner or protected by terrain" and superior cover is "The target is protected by a significant terrain advantage, such as...". Neither of those includes either allies or foes.
Then, later, an exception is granted for ranged attacks that specifically states, 'When you make a ranged attack against an enemy and other enemies are in the way, your target has cover".
Thus, a creature can only grant its allies cover for ranged attacks and not stealth, because creatures are not a part of the terrain and the exception they're granted only covers ranged attacks.
If you are going to argue from the exceptions rule, you should choose the actual rule rather than the brief introductory example description of that rule.
In this case, we want to determine cover. Given that, don't you think it's best if we look at the paragraph titled "Determining Cover"?
"To determine if a target has cover, choose a corner of a square you occupy (or a corner of your attack's origin square) and trace imaginary lines from that corner to every corner of any one square the target occupies. If one or two of those lines are
blocked by an obstacle or an enemy, the target has cover. (A line isn't blocked if it runs along the edge of an
obstacle's or enemy's square). If three or four of those lines are blocked but you have line of effect, the target has superior cover."
Note several things from the rule: 1) Range is not relevant, as I said earlier. Again, can we please discard focus on the term "ranged attack", as it is not relevant and was just using the only possible example (you had to be at range because otherwise there couldn't be someone between you and the target); 2) obstacle and enemy are used interchangeably and as equivalents, and both are used to describe "cover" multiple times.
And on re-reading the cover section, the entire section uses language to describe an attack against a target with cover, and never in the context of stealth or from the perspective of the person with cover. We can presume, if any cover can be used for a stealth check, you determine said cover as normal (IE from the perspective of the foe as if the foe were going to attack the stealthing character ("target")). If not, then no cover rules are applicable to stealth - not obstacles nor people, since both are spoken of from the perspective of an attacker against a target with cover and not a stealthy person in cover making an attack.
Therefore, given range is not relevant, and given determining cover for all cover issues uses a rule that includes both obstacles and enemies being in the way, you can make a stealth check using an ally for cover.