Does this really solve the ethical wicket of human embryo testing?

Subjectively, no. Objectively, yes. Just because it has enough properties to do things similar to an embryo, it has been shown that it is in fact NOT an actual embryo.

Is tricking stem cells into forming an embryo really that different from fertilizing an egg with a sperm cell to form an embryo?

Yes, very much so. Sperm and egg method is you get what you get and you don't throw a fit. Which is less than ideal if a very narrow line of cells is all that you were interested in. Think of the ethical implications of taking a fetal tissue and indicating, "Oh well 90% of this isn't what I wanted. Let's slice that off and focus on this 10% I do want. Oh and freeze that shit I sliced off, someone may want it before it goes bad." The tricking stem cells allows us to focus efforts so that the yield is much higher on what researchers want.

would this still develop into a functional human being if implanted into a womb?

No. It does not. No one has tried with humans but it's been tried with primates. The uterus takes the embryo and plays along for a bit of time but after that, the body figures out the ruse and the whole thing comes apart, usually in fetal resorption. So while this method can produce particular lines of cells quite well, there is obvious things that are massively missing form our understanding of ovum to make this remotely successful. Can we overcome that technical deficit? ABSOLUTELY. Will we? Nah, it's not likely.

Synthetic embryos serve a particular sticking point researchers have about human cell lines. Most governments allow human cell lines to exist for about five weeks (there's particular exceptions to this that have more asterisks than the TOS for most social media sites, I'll not go into them, we're just going to stick to in general here). Thereafter, they must be destroyed. The problem is that if you need a particular line of cells that develops much later in the development stage, you need donor tissue which is much more expensive. With synthetic embryos you can "jump" right to what you need.

So this brings us back to the ethical part of this. Objectively, these cell lines being created by this process come very differently than what we harvest from actual donors. And there's little likelihood that this process is going to develop much further than great for single targeted cell lines, piss poor for complex tissue/organs/actual humans. So objectively speaking, synthetic embryos today have very little chance to be confused for actual human embryos. Today's synthetic embryos are just way too dissimilar to actual embryos that I think any ethical concerns are overblown. Yes, it has the name embryo in it, but that is solely a technical distinction and confusing it with actual embryos is a gross misunderstanding of the details.

Subjectively speaking, if I build a ship out of things that look like wood, act like wood, and feels like wood but is indeed not wood, did I build a wood ship? There's a point where I can make fake wood look real enough that it would be hard to tell if it was wood or not. Likewise, it wouldn't be impossible to develop synthetic embryos to a point that the body would know no difference between it and a real one. The only problem is that much like our wood thing, there are trees that are way cheaper to just grow and harvest than to sit here literally trying to reinvent the tree. The whole sperm/egg thing is just something nature has had a lot of time to perfect and it's going to be a very pretty penny to mimic that. And everyone will find that there are very few takers that want to blow that kind of money.

What synthetic embryos solve is a need for particular lines of cells much later in the development phase of a human life. Those cells are expensive to obtain. Synthetic embryos are a cheaper means to getting SOME of them. But if the goal is an actual embryo, you still cannot beat the cost and effectiveness in your run of the mill fertilization. Additionally, if your goal is large amounts of tissue/full organs, likely that 3D printing is going to beat out this technology but until either one of them wins, we still have the expensive and complex system of being an organ donor and waiting till you get a fatal head injury. So synthetic embryos seem to only be able to serve the niche that they are more affordable than the current method. Could they do more? Oh yeah. Will they? Probably not. It was pretty expensive getting to where they are currently at, and going further there just seems to be better methods for the use cases they would serve.