Are lockdowns justified?

[FlatAssembler]

There is only science, forget the hard and soft horsecock bullshit.

Not all science is equal. Social sciences are, on average, significantly less reliable than natural sciences. Social sciences are dealing with things that are significantly harder to study. And they tend to be done by people, congrationalitions to the exceptions, without much grounding in mathematics and the methodology of science.
And sometimes science is clear, other times it is not.
Science is clear that global warming is real and is at least partly anthropogenic. Is it mostly anthropogenic? Well, that's where science is no longer clear. In order to assume the CO2 we are releasing is responsible for the warming we have observed, you need to assume there is a positive feedback loop between the increase of greenhouse gasses (including CO2 and wate vapor) and the temperature, and that the positive feedback loop increases the effect of CO2 by around 3 times. And the only evidence we have of that are the computer models. And we have no reason to think those computer models are accurate. To the contrary, the vast majority of climate computer models predict the long-wave infrared radiation from Earth will decrease as the CO2 in the atmosphere increases, but the satellite data shows it has been increasing over time. If the climate computer models are wrong about the infrared radiation, why assume they are right about the effects of CO2 on the temperature? And if the current global warming is mostly anthropogenic, what, if anything, should be done about it? That's where the hard science ends, and soft science and politics begins.
Science is clear that LDL cholesterol causes heart attacks, as there are multiple drugs (most famously statins) that demonstrably cure heart disease by decreasing the LDL cholesterol. The claim that saturated fat causes heart disease is probably true, but it is less certain. The claim that the DHA omega-3-acid, found in fish and algae, decrease the risk of heart attacks is... controversial among scientists. The claim that ALA, the omega-3-acid prevalent in the vast majority of foods marketed as being rich in omega-3-acids, can somehow decrease the risk of heart attacks... is probably false.
Computer science is clear that computer malware exists and is sometimes dangerous. It is also clear that any real algorithm for detecting malware has many false positives, which sometimes have very bad consequences. Do antivirus programs do more good than harm... the computer science stops being clear here.
Not everything being marked under science is equally certain. You need to actually look into it to see what the evidence actually points to.

It's not that hard, with the proliferation of journals around, some will publish any nonsense.

Fortunately, however, journals that publish nonsense do not stay for long.

the average secondary school student would beat you for smarts

As far as I know, almost no secondary school student has published papers about linguistics, and very few have about computer science.

You can also get enough vitamin D from

See what I have written below...

Yer an idiot to conclude that lockdowns lead to being lacking in vitamin D and therefore increasing the spread of COVID.

Dude... OK, let's say, for the sake of argument, that I was arguing for forcing people to go vegan in order to protect them from eating too much saturated fat? What would you tell me? You would probably say something like "Forcing people to be vegan without educating them about nutrition will lead to B12 deficiency, which is already common, to increase significantly. Furthermore, given that people who go vegan without knowing much nutrition tend to consume stuff such as coconut milk, the saturated fat consumption may even increase.", right? And that would be a valid argument. And saying "Well, it is possible to be a vegan and not be B12 deficient." or even "Most of the people will not become B12 deficient."... would not be valid counter-arguments. Now, replace "forcing people to vegan" with "lockdown", "Vitamin B12" with "Vitamin D", "saturated fat consumption" with "COVID infections"... and I think you will understand how silly you sound.

That is not correct. Where are you getting your information from?

If 540 thousand Americans died from COVID in 2020, then the excess mortality in 2020 would be at least 540 thousand. But it isn't.