If selected for SLC and not promotable, does that mean something?

It means that you will complete one of the steps to promotion. You will be ahead of peers.

It's not a hidden sign you've made the E7 list. I attended SLC 2 years before making the list. Sometimes they just need to fill slots

They fill slots ensuring the course has the requisite number to meet certain delivery techniques and the like. They do however use a criteria for selection to fill seats, the positive thing is that your records must somehow stand out, this should at least provide you with some hope. Ensure you continue to do the right things. Thank you for your service.

It means GO, attend ANY and ALL schools and training that will make you stand above your peers, teach you new skills or as said below get you selected sooner. As a young SP4 I attended PNCOC/CA then 6 months later BNCOC/CA. I was always in leadership positions earlier than my peers. From what I have heard of the new NCOES it is better to have it done sooner than later. I wouldn't want to be sitting waiting on a school slot for promotion these days. Good luck.

On the positive side, if written properly, this would make an excellent bullet on your NCOER.

If you are not promotable then you shouldn't have been selected for a Senior Leadership Course. In my unit if you failed the APFT or have been taped and failed because you are overweight, then your name would be pulled for any consideration to a SLC. Also you would not be eligible to attend any other schools or be selected for a any exchange program with troops from other countries. It was probably a paper work mistake. If you aren't promotable find out why and make the changes necessary to change your status. If you are a good NCO, someone thought you would be a good choice for SLC and you deserve a chance at advancement. Good Luck!

Yes, someone sees something in your potential

I may not "mean" anything, but it certainly gives you an edge over others on the next list. In the civilian world (now) the more qualifications one has, the more likely they are to be the "right candidate for the job". I see this a probable qualifier for your next promotion (promote ahead of peers should be a bullet somewhere).
Congratulations.