No. At least not any more loosely than any game (D&D) that I play at the table compared to that which is published by WotC. Heck, simple name changes have gotten around WotC IP for years. I know I have purchased more than a few "serpent people" in my time. I don't think that has affected compatibility one bit.
But notice: it refers to "magical" means to communicate with animals because that is expressed as a spell ("You can cast the Beast Sense and Speak with Animals spells but only as Rituals.") Then it refers to the beyond-mortal-limits effects you can achieve with Rage, and calls them "supernatural might", NOT "magic." There's a very good reason for that: some forms of beyond-mundane power are not "magic".
Then "compatibility" means something a hell of a lot stronger when I use it than when you do. "Compatibility", to me, means that you need do nothing, or very nearly nothing, and the work in question can freely use any parts from the official rules that it likes as starting points for its own content.
I always encourage designing your own thing. It is a good exercise, IMO, even if you never actually use it. I hope to get around to my 5e hybrid that incorporates some more 4e isms at some point, but I have been saying that for years. If I do, I plan to have only 4 classes (I think) and handle most everything through subclasses. So you have:
Yay, to my eye that is copying, not compatibility. I definitely have a broader definition than you do!
It has been a long time since I have looked at what they did.
No that seems like a bridge to far, based on your description, but I would have to look at it again more closely. Ideally compatibility is more like any 3PP 5e product (LevelUp, TotV, etc.) and 5e24 are to 5e14.
I don't think this is a good analogy. An "adaptor" can be an incredibly simple approach to still can achieve "compatability" in my view, but it could be something that breaks compatibility too. Take an EV charging station. Most have an adaptor for Teslas and a CCS1 for most other electric cars. You just pull your car up and plug in, easy and compatible. However, there are some electric cars that use neither a Tesla charger nor the standard CCS1. They either can't plug in to those stations (not compatible) or need to add another adaptor (more complex compatibility).
Yes, it depends on what you want, need, and expect. For me, the baseline assumption is that their is some level of adapting done at the table. I never play D&D without house rules / homebrew beyond a session or two. That was the case with 1e, 4e, and 5e. I assume there has to be some adaptation / translation from the published to what I play at the table. So that adaptability is a baseline for me. Simple "plug-and-play," as you seem to be describing it, is never an option for me. That would not be D&D from my viewpoint.
My group has 3 fighters, two rogues, and a wizard. All at level 15, all single classed. I can't speak for others, but my group is having a blast!
I think you answered your own question. @Zardnaar said it was a better "defender" than fighter (which I think they mean "striker" in 4e terms). I personally was happy to see the Slayer in essentials that provided a "striker" fighter type. Really the ranger was your martial (aka fighter) "striker" in 4e. People just get hung up on names. Though, IIRC, the ranger relied on doing multiple attacks to get its max DPR and I did want a single attack DPR leading martial. That is where the slayer eventually came in.
The other issue was that all the weapon based strikers ended up pigeonholed into specific weapons. Rangers were dual weild or bows. Rogues were finesse weapons and crossbows. Barbarian was 2 handed. The Slayer was a striker not tied to a specific subset of weapons, at least at first. (If they allowed rangers to use great swords with their powers at any point, it was after I left).
This. If I see the word “compatible” on something I expect to do some work.
