I mean, I would fucking hope I misread your story, but you did tell this story. Here’s how I interpreted it:

He tried to convince me

I read this as one night in the kitchen with you two having beers at your place, and he says this. Maybe I assumed that wrong? Was it a whole series of conversations, or was it one conversation? If it was a whole series, that’s worse on his part

that my decision to miscegenate was bad for the "white" race

I honestly think this question is key: was it he who brought up the terms “miscagenate” and “the white race” (those three words in that sequence)?

If he used those specific terms it makes him sound more generally racist. But if you’re introducing them for flavor in the story, that speaks to me of a pattern of amplifying your “he’s racist” interpretation in your own mind.

because of a study he'd allegedly read

Why allegedly? You suspect he made it up out of whole cloth? Or did you suspect he only read about the study but not the contents of the study itself?

of wherein mixed race people were found to have a higher risk of suicide.

This, to me, sounds an attempt to be helpful.

He said this to a friend of over two decades, as calmly and confidently as if he'd told me yesterday's weather,

I read this as if he’s not presenting uncertainty, but rather certainty, but he’s only read one study and doesn’t have actual expertise, right?

What exactly did he apply this confidence to? Did he confidently say: “I read a study that said X” or “I know that X” or what exactly?

and apparently expected me to react to this as it it were helpful information.

This is the part that made my blood boil. A parent who doesn’t consider a reported threat to their child, even an uncertain or hypothetical one, to be helpful information just freaks me the fuck out. What?

So, I cut him out of my life,

I read this as you cut him out of your life after two instance of him talking about this study: once to you and once to your mutual friend.

If you did that, after twenty years of friendship, that’s fucked up.

he stalked me

Excellent reason to avoid him. But again I don’t know if I trust your judgment yet.

It seems (unless there’s a lot left out and not even mentioned as ommitted) that one mention of a study involving varying risks to varying racial groups was enough for you to deem him a racist. I guess I misread that? There was more racist stuff he was doing too?

and I fled the country with my wife and toddler in tow

So either you fled the country because of this guy, or you did it for a much larger set of reasons, and you just included that fact to set the overall emotional tone of your family’s story around the same time as the end of that friendship?

All this with the backdrop of a global pandemic and an active insurrection against democracy.

Sorry if I sound too analytical here, but reference to a “backdrop” really makes it sound like you interpreted this guy’s fumbling attempt to help as a plot line in a much larger story about cutting ties and pulling stakes and going on a big journey.