You are prepared to do the skill.
Your flint and steel gets damaged; it's now in fine condition.
You use your flint and steel while making the fire.
You must be outside to start a fire.

Might it be reasonable that using fire building indoors would not consume
flint and steel?

While it makes sense that when trying to build a fire in unfavourable
conditions you would throw everything and the kitchen sink at the issue at
hand, if it is completely impossible (as it is as far as I know) I don't think
a skilled firebuilder would waste their flint and steel.