My own view is that for a DIYer what you have been told is correct. What is called 'polished concrete' is a finish not easily down as a retrofit - search the term on this site and you'll find plenty of info on why. Better to paint, tile (you can get large grey tiles which would give the same colour if the colour is the issue) or use timber of some sort on top - all of which give you multiple choices, varying costs and most can be done DIY.
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.