Tried this solo last night and for the life of me I can't find a mention of defending the space marines in the rulebook. Defence is mentioned on some combat cards so I took it to mean when the genestealers attack.

Anyone else find a defence reference in the rulebook?

I can't remember the exact wording but AFAIR genestealers don't really attack, they just force Space Marines to defend themselves by rolling one die and trying to score anything above the number of genestealers in a stack adjacent to them. So basically Space Marines are the only creatures actively attacking in the game. The genestealer "attack" is just passively forcing marines to defend actively.

So yeah, I'd say you have made the correct ruling. An example might help, though.

That makes sense, thanks. I'll read the rules again to see if I read "attack" when it actually said "defend".