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Sounds like it. It happens with other Google services too. Someone could share a GDoc to your non-Gmail email, but if that non-Gmail email isn’t a Google account, then you won’t be able to access the doc/comments/whatever else (depending on the doc permissions, of course).
I’d consider Signal to be the gold standard of secure communications.
You can describe it to them like WhatsApp, except it’s private, secure, not Facebook-owned, nonprofit so it can’t be bought or sold, etc.
Here’s the blog post that I share with my friends comparing Signal to iMessage and WhatsApp when they ask me about it.
It usually answers most of their questions.