One explanation I've heard for mysterious "We were talking about it in person but nothing else" ads, is that if you were connecting to the internet from the same Wi-Fi access point or IP address as someone else that did a web search on the topic or visited websites on the topic, it has connected you by way of shared internet connection.
Is it possible something like that happened?
In general, while anything is possible, my own occam's razor calculation is that if someone does have a way to get through ostensibly end-to-end encrypted messages, it's going to be government actors saving it for law enforcement/national security purposes. They wouldn't "waste" it on ad targetting. And if it's being secretly used for ad targeting so many people would know about it, people who aren't disciplined military bound by law to secrecy, that it would be quite likely to get out and be revealed and no longer secret.