If you are afraid about your backup seed or private key are being stolen then there is a way to protect them while using this iancoleman tool just download the source code directly as already suggested above or just download the whole page of https://iancoleman.io/bip39/ and turn off your wifi or internet before you use the tool for safety purposes.
And actually, this tool was already created a few years ago and no one still complains about this tool you can anytime check the source code which is open source under their Github page.
Don't compare this tool with Metamask and Trust wallet because they are totally different.
Especially when Trust wallet is mostly closed source and you actually need to trust the company/developer behind it.