I think you are right.

The title of the figure is ambiguous about what "SD" really means but I guess it is plotting the number of standard deviations of the 1991-2020 data measured from the mean of that data and plotted per day for the 1982-2026 data.

Here's the link that I read off the figure.

https://climatereanalyzer.org/clim/sst_daily/json_2clim/oiss...

Going up the URL path I get redirected to here:

https://climatereanalyzer.org/clim/sst_daily/?dm_id=world2

That shows measured temperature and 2026 as very hot (surprise) and in fact is the hottest to date for June+July. The standard deviation of that data is not 3.5 C but something less than 1.0 C. It is plausible that current temperature is about 3.5 sigma from the selected mean.

It's worth recognizing that the analysis is applying a biased conclusion prior to making the plot. It singles out post 2020 data to compare to pre-2020 data. and then concluding the held out data is a significant deviation with a cause. It almost certainly is but this is not a proper way to analyze data (unless one is pushing an agenda, be that for good or bad).

I think the sst_daily plot stands on its own without crafting this SD plot to emphasize the point. Especially when the accompanying text doesn't even explain it. It's a disingenuous message.