Do "water repellent" coatings such as the Hoya HD3 or EVO filters prevent condensation/dew?

Question

As asked in the subject — do they help? do they entirely eliminate the problem within "reasonable" humidity levels?

In the manufacturer's description, they only emphasize the aspect of being easy to clean, nothing on any effect during usage.

Answer 1

Condensation is only an issue if the glass is much colder than the surrounding air, until your reach dew-point. Dew point is humidity-related & seasonally-variable, but you could roughly call it 4° in winter up to 14° in summer.

OK, so below these temperatures dew can form on a surface the same temperature as the air.
On a water-repellant - hydrophobic - surface there is the possibility it may not get chance to form if there is sufficient wind movement, but actually what is equally likely to happen is that it forms in very well-defined beads rather than smears & these run off in clumps. You can see this on your car in the morning if you've recently polished it or have water-repellant coating on the windows.

This is fine on a car, as once you're moving it will quickly shed & look shiny again. On a lens, not so much.

So, what you actually need is not hydrophobia, but hydrophilia. You want a surface that actually likes water.
Back to the car reference again; this is what you put on the inside of your windscreen. If you ever tried putting a hydrophobic product on by mistake [as I did a couple of years ago] you quickly learn what a colossal mistake that is. It generates more run-off water & massive droplets than you could imagine possible inside a car. A hydrophilic substance makes each tiny droplet spread, so you can't see it, & less likely to form in the first place compared to a hydrophobic surface [I have no idea how that bit works, but it does].

So, someone invented a solution - the Tokina hydrophilic filter.
Great, we all thought.
Unfortunately the results were OK in daylight but abysmal at night & they were discontinued. You can still find some new on eBay etc if you wanted to try one out.
See https://www.newsshooter.com/2015/07/22/no-more-rain-drops-on-your-lens-the-tokina-hydrophilic-water-dispersion-filter-reviewed/ for a good review & test drive.

So you're left with a hydrophobic filter, or none at all & giving it a wipe every so often; or indeed, some car windscreen anti-fog rub-on. I've never tried it on a lens. It needs care on a car windscreen & has a tendency towards being smeary unless you rub it down really finely. I'd test it on a cheap filter rather than your good stuff, if you ever wanted to experiment.
[The other answer also includes a tip so staggeringly obvious I didn't even think of it;) Warm the lens….]

Answer 2

Since you placed an astrophotography tag on your question, I'll assume you are experiencing fogging while shooting the night skies.
You'll get fogging/beading even with hydrophobic coatings as Tetsujin stated. Problem is, the front lens element is being cooled by pointing it up at a cold sky (radiative cooling) which can cool the front lens element below the dew point. This is the same reason why you get frost on car windscreens that see the sky even when the air temperature is a few degrees above freezing.

Common solution to this problem is place a warming jacket around the front of the lens to keep the front element temperature above the dew point. This can be a purpose-built battery powered unit or wrapping a couple disposable hand warmer packets around the lens (hold it in place with a repurposed sock that has the toe removed).