1. Are link only answers accepted here on SuperUser?
They are accepted, but they are not classed as VLQ.
VLQ (Very Low Quality) is defined as "This question has severe formatting or content problems. This question is unlikely to be salvageable through editing, and might need to be removed."
A link only answer can be salvaged through editing, and the comment that was added is correct. If the OP follows the advice it becomes a good answer (and so it is not a VLQ answer).
If the answer is downvoted, it will appear in the "Low Quality Posts" review queue where users with sufficient reputation can vote to delete if the answer has not been improved.
2. The reason given for rejection is declined - it's a non-answer and use that flag.
Again this was a link only answer link for 10k+ userslink for 10k+ users.
See above for why this is not a VLQ answer.
NAA (Not an Answer) is defined as "This was posted as an answer, but it does not attempt to answer the question. It should possibly be an edit, a comment, another question, or deleted altogether."
In this case the link just said "Ubuntu" so if the link fails it becomes "Not an Answer"
The rest of the answer said "So I am not really into Ubuntu but I think this will answer your problem.", which is an indication that the OP is not really sure and is just guessing. Such answers really should be comments, which fits the NAA definition.