It's a good idea to have that policy and be careful.

I submitted a comment in a similar office survey service and despite my submission being anonymous, shortly after submitting it, a superior had a meeting with me to address what the anonymous complaint was about, having determined the comment was from me.

In this case I was very glad they figured it out because it ended up being a low risk way of bringing up my concern, but had I not had such a great employer, the result could have been me getting fired or something similar.

The comment was only one short sentence and there were 50-100 people employed at the company, so if you want to be honest on platforms like this, I would limit what you say and obfuscate as much as you can. It doesn't take much to identify you.