Dot.TK stores passwords in plain text [png]

In their signup process, they might grab the pwd you entered in the registration form, generate the email, with password. Then send that email directly without storing it, while at the same time, inserting your password with an encryption into a database. Is it a good idea to send passwords by email? Well, no, but that's a different story. I don't know the actual implementation so you might be correct. But this isn't proof. What kind of email do you get if you need to reset your password by the way?

It is proof that they are not using a non-reversible hash to store it in their database.

They could be storing it using a reversible encryption algorithm, of which there are many, and they are not considered "insecure".

Or, in the worst case, they could be storing it unsecured. It's definitely a possibility.

It's poor practice, for sure, to email someone a plaintext password, as email itself is prone to numerous attack vectors.

But given the overall lack of evidence and insight into their back end, you don't have enough to draw either conclusion.