It is but it was a reasonable leap on Quake - Quake had 256 colour palette for the whole game whereas Half-Life had a 256 colour palette per texture, Half-Life got coloured lighting, Quake had game code written in QuakeC whereas Half-Life had game code written in C/C++, Quake had vertex animations but Half-Life had skeletal animations etc.
Definitely the same bones and I'd say that a lot of engine code would be identical, but there were some big changes that aren't trivial to implement in id Tech 2 to get feature parity with Half-Life.
