If you saw it on a camera that you previously disclosed to the sitter, it’s not creepy (or, at least, it’s no creepier than any other bit of today’s ubiquitous surveillance state).
You appear to be stuck between a rock and a hard place here. It’s terribly irresponsible of your sitter to let your dogs out unsupervised which, imo, trumps any creepiness about how you know that.
You have contacted THS already and received no useful assistance. The THS helpline is reported to be very hit or miss and may require that you escalate the issue. Have you tried the Member Dispute Process ? I believe it can be done entirely online which should be less of a time sink than getting on the phone.