The lady has no reason to apologize for being fit. More people could do it if they really, really wanted to. I know women just like her who you would never believe have had children and they look great. It’s not impossible to do.
If you take offense, the problem is with you IMO.
Women typically feel fat and unattractive after giving birth. It’s hard work! I think that by showing herself so fit after three kids she’s actually trying to give them encouragement.
Some of us, no matter how much we diet and exercise, will never look like that.
I am very jealous of women who don't have stretch marks after pregnancy. Most women, however, get them.