A wide ranging Interview with the Verge on Half-Life: Alyx, confirms that this is in fact the next part of the Half-Life Story, rather than a "spin off"
"Half-Life: Alyx is set in the aftermath of the Combine invasion of Earth, before the return of Gordon Freeman in Half-Life 2. While it does take place before the events of Half-Life 2, we actually recommend that you play through Half-Life 2: Episode 2 before you play Half-Life: Alyx, for reasons that will become clear as you progress. It's not a side story, nor an episodic one — it's the next part of the Half-Life story, in a game around the same length as Half-Life 2. "
"Half-Life: Alyx is set in the aftermath of the Combine invasion of Earth, before the return of Gordon Freeman in Half-Life 2. While it does take place before the events of Half-Life 2, we actually recommend that you play through Half-Life 2: Episode 2 before you play Half-Life: Alyx, for reasons that will become clear as you progress. It's not a side story, nor an episodic one — it's the next part of the Half-Life story, in a game around the same length as Half-Life 2. "