Meaning of ‘or lack thereof’

The passage below is from Jellyfish age backwards by Nicklas Brendborg.

The similar lifespans of spouses probably stem from the fact that we tend to marry people who are somewhat similar to ourselves. Obviously, we don’t know the life expectancy of our future partners in advance, but they are likely to be people with whom we share interests in things like diet and exercise (or lack thereof), and who have similar levels of wealth, and similar physical traits.

In this passage I want ask the meaning of ‘or lack thereof’.

In dictionary 'the lack of it' means ‘the lack of the thing(s) mentioned before’.

I think I know its meaning. But when I try to apply this meaning the sentence in question, a problem comes along.

Let me explain my problem.

Here is the sentence.

‘They are likely to be people with whom we share interests in things like diet and exercise (or lack thereof).’

I paraphrased based on its literal meaning.

‘They are likely to be people with whom we share interests in things like diet and exercise (or the lack of diet and exercise).’

Am I right?

Then does ‘diet and exercise’ before the parenthesis mean not just ‘diet and exercise’ but ‘eating much and doing exercise a lot’ so it could have contrasting effect with ‘the lack of diet and exercise’.

Am I right?

Thanks in advance.