No, they're saying that unless you're already good at this stuff, it's easy to assume that a visually 90° angle is actually 90° even when it's not
Especially if you are bad at this stuff you know that drawing anything like that accurately is a real pain and nobody who can avoid it will ever do it to represent anything accurately. That is what labels are for.
People tweeting stuff. We allow tweets from anyone.
RULES:
No, they're saying that unless you're already good at this stuff, it's easy to assume that a visually 90° angle is actually 90° even when it's not
Especially if you are bad at this stuff you know that drawing anything like that accurately is a real pain and nobody who can avoid it will ever do it to represent anything accurately. That is what labels are for.