You probably aren't wrong for your baby. She probably isnt for hers.
My middle two we had a water tight routine. It was stuck to with iron discipline. Because all hell broke loose if we didn't. They also napped till well after their 4th birthday.
My youngest.... well, she's incredibly flexible. She doesn't do better or worse on a routine as long as she gets enough sleep in total. I've never known a toddler to genuinely not seem to care about when she naps! During term it's most convenient if she has 1 2 hour nap at midday. She is happy with that, goes down without a fuss and wakes up happy. Yesterday we were out on a day trip. She slept in the car at 5.30-7. Still happy all day, went to sleep easily and woke up happy.
I think most toddlers do better with a routine, but perhaps hers is one of the rare ones that just doesnt need one?
Equally, as regards total sleep - my eldest has never slept much. By the time she was 2, it was a total of 10 hours in 24. She's 8 now, she sleeps about 8-9 hours. She has never slept more unless ill. Yes, it's a bit unusual, but it seems to work for her. I don't sleep as much as is recommended either, never have done. I'm very satisfied on 6 hours generally (and at the moment I bloody wish I got that!!!)
If her child is happy and thriving, then great.
Perhaps a little more concerning is the diet. You mention they are low on centiles etc. Are the family generally small? But again, there may be more to it - my eldest is much slighter than my others because she is a picky eater. On holiday or with guests I would allow her to eat "worse" food than at home as i wouldn't want her food to become the focus of quality family time. So yes, there would be a lot more chips than at home but I wouldnt worry about it.