meaning of "sold out for good"

If someone sells out of a product, that means they have sold them all, so there are no more left. It can also be phrased as “We are out of the product.” I can be temporarily (We are sold out for now.) or permanently (we are sold out for good)

Please explain this paragraph.

.
What part do you not understand? The phrases in parentheses paraphrase the preceding phrases.
.

Particularly (the bold group of words)

Also, in this (If someone sells out of a product, that means they have sold them all, so there are no more left)
Here, someone sells out of a product looks like present tense
But its explanation (that means they have sold them all, so there are no more left) looks like past tense

So, here also I am confused.

.
I agree that the tense use is rather sloppy in the excerpt you have presented to us.

‘We are out’ = we have no product remaining
‘We are sold out for now’ = we temporarily and at the moment have no product remaining
‘We are sold out for good’ = we have no product remaining and will never have any product until the Universe ends.
.