I've read somewhere that they get gains without applying progressive overload. That would mean one could get bigger muscles without getting stronger, because he doesn't add reps/weight each workout.
no.. progressive overload isnt absolutely necessary for a newbie. although youll stop gaining after a 3 to 4 weeks and it is highly inefficient.
As a beginner, you'll make gains quickly in both strength and muscle with little effort because your body is suddenly going from couch potato with fatty diet to using your muscles everyday and eating clean.. essentially getting in natural shape. Technically this is progressive overload because your going from doing no exercise, to doing exercise such as bicep curls no weights.. or eve doing exercises with weights such as bicep curls but with 5 lb dumbbells. To answer your question, no. progressive overload is not needed. although your gains will be slower and much less significant and your body will likely stall within a few weeks due to not needing to add muscle or strength to lift the same weights. thats the definition of progressive overload because your body does not need to enhance itself to lift the same weights unless your lifting them for more reps or lifting more weight.
if you do this you will likely not get in shape and gain maybe 2 to 5 lbs at best of muscle.
After 6 months to a year the newbie phase ends and youll have to change your diet and workout. Athough if youre not using progressive overload, your gains will probably stop within a few weeks. I dnt understand why you wouldnt want to increase the weight or add reps unless youre trying to be as lazy as possible with a workout
That would mean one could get bigger muscles without getting stronger
I think you've got this backwards.
There are two ways in which you get stronger: add more muscle fibres, or, increase your muscle fibre recruitment (i.e. how efficiently you use your muscles).
When you workout without a progressive overload and without eating in a surplus you are increasing your body's ability to recruit muscle fibres. This is the opposite of your statement: you'll get stronger without getting bigger muscles.
You said that you trained to failure every day. What did your reps start at, and what did they end at? If you started at 10 and ended at 20 that is, by definition, progress.