A lot of things about Lp(a) are still hypotheses
Lp(a) appears to be an anomaly in the structure of a protein on the surface of lipoproteins you make that causes them to be difficult to clean up when their job is done. It’s an inherited trait that causes you to make more anomalous than normal ones. It’s essentially an LDL with an Apoplioprotein A on it’s surface that shouldn’t be there, and it has a long tail that your cells can apparently be mistaken for an inflammation repair material called fibrinogen.
Normally after a certain time in circulation delivering lipids, our lipoprotein particles acquire a protein (Apolipoprotein E) on their surface that allows them to be taken out of circulation (70% by the liver 30% by other cells). The longer they are in circulation the weirder they get - oxidized, or glycated - the blood they travel in is a very reactive environment full of dangers for lipoproteins.
So anything that causes LDL to stay in circulation longer is bad. Lp(a) makes it more difficult to pull out of circulation. So your lipoproteins hang about longer and tend to be weirder than most.
That “clear me” protein comes from your energy carrying particles (Chylomicrons and LDL) bumping into HDL particles, which may be why the more HDL you have the better your health. Even if you make a lot of Lp(a), the more HDL you have the more chances it’ll get taken out of circulation in a timely fashion. So maybe a diet that promotes HDL is good for people who make Lp(a).
When you have inflamed blood vessel walls, your body will recruit resources to fix that from circulating LDL and if your LDL is weird … you’ll apparently get unstable repair jobs in the wall of your arteries. And that is likely one of the causes of atherosclerosis, which causes thrombosis and occlusion around the vessels feeding the heart, or brain and … well that’s bad news. The fact that these specific LP(a) particles are also mistaken for fibrinogen means they are more likely to be found around blood vessel repairs.
The fact that cell wall inflammation can be caused by high levels of insulin also means a diet that causes you to make less is going to be better for people who make Lp(a).