Prune juice not for under 9 months?

DD has been a bit constipated the last few days, I gave her a little bit of prune juice this afternoon (which she seemed to love! I thought I'd have trouble trying to get her to swallow it) to try and soften things up a bit as the poor thing has been in so much pain trying to poo...
Have been talking to MIL about it and she just sent us a link to a vic health website that states prune juice isn't suitable for babies under 9 months? Everything else I have read has suggested using it to help things along, not that it can be detrimental to them (the link says it contains a bowel irritant or something and therefore is not suitable to help bubs with constipation issues). Is this right as nothing else seems to suggest this??

She would have only had a few teaspoons so I'm sure she will be fine either way, but can I keep giving a little bit to her at least until her poos go back to 'normal' again?

my GP suggested this at 6 months for DD so i have no drama giving it to her - i do water it down though - 1 part juice, 2 parts water. given she was going anywhere up to 10 days between movements before, and now we've started it's gone back to only 3 or 4 days. she still isn't pooping the way she used to be as a little little bubba, but she is doing much better now...