They may well be looking at how much the EU holds in Saudi assets, seeing those at potential risk of being seized and deciding tge write-down on dumping the bonds would be worth it. Long term, I don’t think it would have an effect on prices, but short term it may well do, depending on how concentrated their holdings are.
From what I can see, normal trading volume in bonds is about 500% per year, or about 2% per day assuming 250 trading days per year. If the 130bn you mention is spread across all government bonds across the EU then it accounts for about 4% of the total, or about two days of normal trade. Dump all of that in one go and it’d definitely have a short term effect. If their holdings are more concentrated they could have an even bigger effect on the bonds they hold.
Bonds tend to be issued on a regular basis, so even a short term drop in price could be timed to affect an auction. That has the twin effect of reducing the amount the government in question raises, and also tying them into effectively higher interest rates, potentially for decades to come.
I’m no expert trader either, so I could be barking up the wrong tree, but I assume that they would have a clear expectation of the results before making that threat, and I can’t really see any other effects it could be expected to have.