For many, both exercise and healthy eating are necessary to lose weight.

got it under control until I started going to the gym and biking.

But typically:

Better “Kitchen work” can help reduce caloric intake.

Better “gym work” builds muscle tissue which in general burns more calories even while you rest.

It’s a balance that each person needs to discover what ratio works for them.

Kitchen = eating habits
Gym = activity

For me both would work if I get off my arse at some point

yes indeed

A similar saying I’ve heard is that a six pack isn’t built in the gym but in the kitchen.

169 lbs without working out, walking or running. I ate nothing but chicken, eggs and cottage cheese.
I would never recommend it now.