It sounds like tennis is her main 'hobby,' and most of the others are just socialising that happens to be based around sport.
Snowboarding sounds like basically a dad/daughter spending time together. Yes it involves an hour of snowboarding, but would the same people be complaining if they did something different each weekend? e.g. if you said 'On Sat DH takes DD to the cinema for a few hours.'
As a pp said, there does sound like there's some sexism as well. If it was a 'DH takes his DS for a kickaround in the park and then for lunch every Sat' I bet you'd get nothing other than 'aw how lovely for them to spend that time together,' even if the DS also did 6hrs of footie clubs in the week.
Same with gymnastics - she's mainly doing it to see her friends. It's probably not more energetic than if they just met up in the local park and ran around or cycled around on their bikes as would have been more normal 20 years ago - it just happens to be a structured activity which most of them will naturally drop out of over the next few years.
All kids are different - as long as she's happy. Presumably you wouldn't dream of commenting out loud 'what your kid NEVER does any exercise and sits watching TV for hours a day? He must be so bored, unhealthy and unstimulated,' but the reverse is okay to say to you?
Being honest most parents don't let their kids 'learn to entertain themselves when they're bored.' If they have siblings they might leave them to 'entertain' each other (which isn't an option for you), but even then in my experience it involves playing mediator every five minutes, and only works if they are fairly close together in age and interests. Lots of siblings have nothing in common. I bet lots of the parents saying this to you just switch the TV on and let that do the entertaining.
I imagine by the time she's in secondary, the tennis and maybe harriers will stay as her main hobbies, she won't bother with gymnastics and will just hang out with her friends independently instead, and she'll get to a level where she might no longer need/want snowboarding lessons every week but it could still be more of a fun thing she and DH do on holiday/once per half term or whatever.
Until then, unless she shows any signs of being too exhausted, let her be. It would be ridiculous to stop her doing things she enjoys so she can sit around bored just to make a point.