To be fair, very few tech companies would hire someone who starts off by expressing their support for unionization. It's certainly a good "unsubscribe" tactic. For example, if you want to get rid of unwanted Linkedin outreach, just put into your title "Go unions!"

Being against anti-union tactics involving privacy and surveillance issues is not the same as being pro union.

Employers won't hold it against you for being against tactics that involve privacy and surveillance issues in general (eg: "I wouldn't want to work at a place that spies on every single Slack message I send to coworkers"), because they give you the benefit of doubt that from their bottom line perspective you will be a net-positive contributor. The minute you include the word U-N-I-O... (eg: I wouldn't want to work at a place that spies on unionization efforts), that benefit of doubt is lost, and now everyone is wondering why that topic is on your mind.

None of this is a reflection of the value of unions from the employees' perspective. But it's important to realize that unions are one of the instances where the employees' and employers' interests are not aligned. So, emailing a recruiter and mentioning unions has only one possible outcome - none.

God, America is weird.

Sounds like you both dodged a bullet.