IF you already have an email domain you control.

Calling "acquiring and setting up an email domain and configuring the mail server for wildcards" "basically no extra effort" is a bit disingenuous compared to "solve a captcha for a Gmail account"

Spamming user+1@gmail, user+2@gmail takes absolutely no technical knowledge whatsoever - anyone can do it with 1 gmail account.

Spamming user1@domain, user2@domain etc requires 1 of two things:

1. you can sign up for multiple email accounts using a third party service. You're going to run into trouble with Gmail or other big providers if you start creating accounts en masse.

2. you create your own email server. this requires someone with selfhosting knowledge and some basic coding (or rather server config) experience.

it takes basically no extra effort

I'd assume one needs to verify the email by clicking a link, so to spam user1@domain.tld, user2@domain.tld would mean you need access to those inboxes. That means you need to go through the effort to actually create those emailadresses on whatever freemail service you chose, or you need to host the emailserver yourself and have all mails run into a catchall inbox.
Hosting your own emailserver is definately not "basically no extra effort", even for a lot of tech-savvy people, paying for a hosted email service using your own domain is easier, but also seems like not a good investment just to spam a petition website.

The foo+bar@gmail.com functionality, however, is pretty well known tool - even by non-tech savvy people. Even some people I know that I consider basically tech-illiterate have known this for years, they have told me when they found out about it and asked me if I was aware of this functionality.

The first one I mentioned requires preparation, setting up email accounts or an email server, the second one is basically already set up for most email users and ready to go, the latter is therefore definately a lot less effort to pull off.