This piece of core reading smells like it was taken from regulation:
https://www.eiopa.europa.eu/rulebook/solvency-ii/article-2457_en
So the short answer is: “the regulation says so”.
To be honest just saying “swap rate” doesn’t tell us which swap rate either. Although the liquid swaps for most currencies are now the OIS instruments rather than LIBOR based, so using the OIS swaps to build a risk free curve would make the most sense to me. I vaguely remember life actuaries arguing about this kind of thing when I worked in consulting, but I worked in GI at the time so wasn’t close to the issue.
I can think of several reasons to use a swap curve instead of a bond curve. Bond prices might have weird effects distorting the curve.
For example Japan is currently controlling their yield curve by buying (or selling, but mainly buying) lots of government bonds to keep the 10y point of the yield curve within certain bounds. Meanwhile the swaps are trading in a more normal fashion.
A bond issue might trade at a discount or premium (so decrease the rate) if it becomes the cheapest-to-deliver against a bond future. (Google cheapest to deliver if you’re interested, I don’t know if the syllabus mentions this.)
A bond issue might trade at a premium if there is exceptional demand for it in the repo market. (Again, can Google if interested.)
The bond market is just a bit more idiosyncratic, you might say. So when you build a risk free curve using bonds you tend to need to smooth it out a bit as well. This makes it more subjective - different people might smooth it differently.
Using swaps is a bit more objective in the sense people tend to end up with a more similar curve to one another.
Some of this might be why the regulation says to use the swap curve if you can.
I guess it could give some weird effects if you hold government bonds as “risk free” assets though, because changes in the spread of swaps to bonds will cause assets (bonds) and liabilities (discounted at swap rates) to fluctuate by different amounts.