Couldnt wait: I just made a random address and pasted the priv key in what you found. I got this:
(...)
Seems legit?
Well, TBH, I have got little experience with eth, and I have never used this specific interface I found. My fluency in coding is not enough for me to take my own conclusions on it, from their open source code on github, but the comments I saw on the reddit seemed fine enough...
If I were you, though, feeling insecure, I'd try to find alternative (safer) solutions on eth specific forums (where help will be plenty, for sure). But, if there's not other way, the risk involving this solution I've found would be better that no solution (i.e. losing the coins anyway)