The SOLID principles of OOP is quite an advanced topic that even seasoned developers tend to miss when writing their code. I personally watch the tutorials at 1.25x or 1.5x speed with no problems but this is also because I have 8 years of prior Java experience and Computer Science background.
Dependency Injection is a rather deep topic on its own and maybe way over the head of those taking the Intermediate classes even if explained in full technical detail. I understand why he brushed on it but did not dwell on it. That was a good choice actually.
The bottom-line of Extensibility is that any changes you make in your code should have little to no impact on existing code, with preference on the latter. An interface lets you do this by creating classes implementing that interface. This makes your constructors or methods highly flexible because they can accept classes that simply implement those interfaces, rather than relying on concrete classes that can change any time.
Having said that, strictly typed languages like Java or C# are known to have steep entry barriers and rather feels unforgiving to many beginners (myself included long time ago). You are not alone. Hence why the fast and easy reward of Python which is more forgiving in its semantics and syntax can be more attractive to starting learners (nothing against that).
I do agree it is bit of a shame that Mosh does not directly participate in his forums. In the real corporate world though, you will encounter senior developers with serious elitism and 'god-complex' attitude problems. Mosh is by far, a gem in comparison.