Ugh.
It's so frustrating when someone who rarely sees your child makes half baked statements like that.
Growth charts are meant to be looked at as a trend over time - not one off percentile readings like that. Honestly you'd think that GP's would know a bit about the growth of breastfed babies but alot of the time they have no idea.

Glad to see she didn't undermine your confidence though. Keep up the good work