It's not "leftists made me turn right" in all cases, it's also formerly progressive points (equality instead of equity, freedom of speech, science over safetyism) shifting into being seen as right-wing positions. The position didn't change, the label did.
As to whether this is the case with Weiss, supposedly not. I believe you.
Radicals may not be responsible for the actions of opponents, but for sure they are accountable. Or at the very least deal with the predictable back-lash.
This is just the nature of these things. They aren't fixed. They're descriptions of views of different groups of people at a certain time, and change with time.
My childhood overlapped with the aftermath and social remnants of jim crow. Back then, there, a moderate position was that integration was inevitable and necessary for technical constitutional law reasons and to prevent wider upheaval, but that it should not have been welcomed and was allowed to progress too quickly. Only a radical leftist would hold that black and white people were fundamentally equal and the law and social norms must be immediately brought to respect that.
Within my memory, a centrist position was that gay men should be medically treated for that "disorder" rather than pursued criminally for it. A radical leftist position was that they should be allowed to openly participate as full members of society without restriction.
You're right that what is considered left or right changes. Center positions do often become far right positions with time. Sorry lol. It still isn't the fault of leftists.
In reality it's much more of a mixed bag. Some ideas are impractical, downright destructive or achieve the exact opposite.
Unappealing unhinged ideas that alienate large groups and act as the perfect recruitment tool for the right wing? Absolutely the fault of leftists.
But agree to disagree, this isn't the place.
I don't know anything about the specific situation you're alluding to so can't responsibly comment. But "the leftists made me turn to the right" has been an excuse as long as I can remember and it's a thin one. Radicals simply aren't responsible for the actions their opponents choose to take.