Do you know how they calculated this? Is it just 3P FG% versus expected or do they take defense into account in some way?
That's my twitter handle, so I do have some idea, yes. It's just opposing 3PT% vs. league average. It is based on research
such as this showing that opposing 3PT% is not meaningfully predictive of itself. The best teams in opposing 3PT% over the first half will tend to regress to league average in the second half, and the worst teams in opposing 3PT% will likewise regress to league average.
This is not the same thing as saying three-point defense doesn't exist by the way - team
do vary substantially and predictably in allowing three point attempts in the first place. Good three point defense is largely not allowing those looks in the first place. Poor defense like you mention would partially be captured in those in extra three point attempts allowed by the Suns.
I would analogize this to pitcher BABIP, where more granular data (such groundball percentage and infield fly rate) allows us to predict pitchers within a range of BABIP (say .280 to .305), but very large samples are needed to make those predictions, and raw BABIP by itself isn't much help.
Put another way, I'm sure opposing 3PT% isn't actually pure random, but I don't think merely having a bad opposing 3PT% over 3, 4, 5, or 40 games tells you very much about where it will be for the rest of the year. As such, without doing a more granular analysis, your best bet in projecting opposing 3PT% is to peg it to essentially league average.