Boys are not being treated like defective girls (because girls shouldn't be treated that way either).
One common idea floating around the manosphere is that modern boys are being treated like defective girls.
Presumably this is because the way that boys and girls are raised as they grow up is getting more and more similar as the generations go on. And correlated with this passing of time, boys are doing worse in school compared to their female counterparts. And the manosphere likes to crow that boys are being treated like defective girls and that therefore boys specifically should be treated in a different way.
However, this increased floundering among boys is not because they are being treated like defective girls. It's because no one should be treated like girls are (and no one cared until boys started being treated that way too).
Maturity
Little girls are widely considered more mature than little boys.
A reasonable explanation for this is that, in aggregate, girls take fewer teachable moments and reinforcements to internalize the various life less. What takes a boy 20 instances of reinforcement to understand might take a girl 17. What takes a boy 10 might take a girl 6. And so on.
However, little girls are expected to internalize life lessons after the first time they're told - if they're told at all. They are not given any lenience, guidance, or understanding when they make mistakes and faux pas. Instead, they're treated like they're malicious entities since they're "more mature" (and should therefore already know that what they're doing is bad), and therefore know exactly what they're doing.
This is a terrible way to treat people, regardless of gender. And while it sucks that boys are being treated this way too now, girls shouldn't be treated this way either.
Expectations
Because little girls are supposedly so mature, they are expected to exhibit the social cognizance of fully grown adults before they hit middle school.
If a girl forgets an important date, or her homework, or that certain topics are off-limits to certain people, she's punished accordingly and given no consideration or guidance on how she can do better in the future. She has to figure it out entirely on her own, or she will keep getting punished until she does. Because she's a girl and therefore should know better.
And all kids (in the US, at least) are expected to sit down, shut up, and stare at a whiteboard for 8 hours a day. Recess is almost nonexistent, and these young animals (since humans are animals) get very little opportunity to release the pent up energy they have. This is beneficial to no one, but kids have little choice but to shut up and do what they're told.
But since little girls are less disruptive in class than little boys, and are less likely to make their issues everyone else's problem, society has declared that these little girls are not only not floundering, but are instead thriving! Which is an insane conclusion to draw, but not surprising since it's only male-leaning metrics that are considered when determining how well a kid is doing.
But of course, these expectations are completely unreasonable to put on kids, regardless of sex.
Dysfunction
As should be surprising to no one, holding little girls to unreasonable expectations, treating them like they're horrible people for making honest mistake, and expecting them to act like adults with no actual training on how to do so leads to a lot of emotionally dysfuctional women.
However, rather than recognizing that this emotional dysfunction is a result of treating them terribly, this dysfunction is considered a feature of being a woman. And since this dysfunction is considered inherent to being a women, it's not considered something that can be changed.
But when little boys develop the same emotional dysfunction, it's suddenly a problem that needs boy-specific solutions. Which is unfortunate since many of the proposed solutions aimed at helping boys now that they're struggling (like extra recess or starting school a year later) are things that could help girls too.
Treating kids the way we treat little girls will often result in dysfunctional adults, regardless of what gender that adult is. Little boys being treated the way that little girls are treated doesn't mean that boys are being treated like defective girls - it means that they're being treated poorly. And all kids need to be treated better - not just boys.