Some plastics can leach chemicals which can have toxic effects on the body.
But overall this question is silly. "Toxic" can be defined as "capable of causing injury or death, especially by chemical means; poisonous". So even if it's not necessarily chemically or poisonously bad for the body, clogged arteries can cause injury and death via heart attacks and or strokes regardless of what is clogging them. So unless someone is trying to argue that microplastics found in the clog didn't help contribute to the clog to any degree it's clearly having a bad effect on the body.
So this is like trying to debate if a stainless steel knife found in someone's heart could have had a "toxic" effect on their body.
The study that is linked to in the article did find that "patients with carotid artery plaque in which MNPs (Microplastics and Nanoplastics) were detected had a higher risk of a composite of myocardial infarction, stroke, or death from any cause at 34 months of follow-up than those in whom MNPs were not detected."
That should be enough to make a valid claim that microplastics can be "toxic" given that their presence has been correlated to higher risk of injury or death. Then there's also knowledge of how they can leach and pose certain risks to people and ecosystems and so on.
I think the problem here is that it's already mostly known that they're probably bad so that's not very news-worthy, but yes there's no 100% conclusive evidence that they're the direct cause of harm yet. It's like the state cigarette smoking was in before there was 100% conclusive evidence that it causes lung cancer. Sure, there was already plenty of evidence that it was clearly unhealthy, it clearly contained various unhealthy things that would obviously have unhealthy effects on the body, and it was correlated with higher risk of death... but it didn't make the big headlines until it did get that 100% conclusive evidence. And just like cigarette smoking mircoplastics have trillion dollar corporations that will use their bags of money to delay, confuse, and obstruct efforts to reduce microplatics because they want to continue profiting off their products so IMO any statements like "we won't know until there's 100% proof!" should be taken with many grains of salt. There's already more than enough evidence to know that the sooner things are done to decrease the spread of microplastics the better.