The point of the article is that the phone numbers that robocalls come from are trivially spoofed. So, it's plausible that these are real people's phones being spoofed, and it would be quite rude to them to publish their phone numbers in a way that could be read as indicating the number is a robot.
So it's preventing the equivalent of dialing random numbers and hoping it's a real person?
No, but it could be preventing the equivalent of "here is a random phone number I picked, please 1000 of you dial it".
No. When you get a spam call, the caller ID is often not random. It's usually the number of some other person on the spam caller's list of people to call, often someone from your same area code. That way, you see a call from your area code and are more likely to answer it. If you get upset and call it back, you won't be calling the spammer. You'll be calling some other innocent victim of the spammer.
Most of the time the caller id shares the same area code and first 3 digits of the recipient's phone number. Many times the last 4 digits are the same just in a different order.
Because spammers populate the caller ID with other people's phone numbers. For example, a spammer might make the caller ID show your (or my) phone number. The caller ID that shows up when a spammer calls you is usually useless.
Over the past year or so I've received two or three irate phonecalls from people telling at me to stop robocalling them, unaware that my number had been spoofed.
Probably to avoid tipping off the robo-callers' authors
Tip them off in what way? They couldn't possibly do anything meaningful.
In theory, since the numbers are essentially made up for each outbound call, the combination of spoof source numbers and call times would be enough for some robocaller/s to correlate with their outbound call logs and discover which number it was that they dialled that reached the CEO of Twilio. That seems like useful info?
In fact the timestamps and partial numbers are probably good enough for that.
They're often spoofed numbers of actual real numbers.