The reason why long-lasting serious acne leaves scars is because acne causes intensive, long lasting skin inflammation and intensive chronic inflammation eventually damages the skin, it results in skin atrophy: Indented scars.
The missing skin of the intended scar has to be rebuilt and this can take a long time.
Collagen production is a slow process and it goes through several stages. It can take months to see results.
Softening of scar edges (by crushing the scar tissue) after one dermarolling/needling session is certainly realistic. If the scar edges collapse, the scar will become less indented.
But I cannot tell you whether this happened in your case or not - it would be nice to get feedback from you, how the skin situation continues.
We have customers whose scars improved significantly after just one needling/rolling and we have customers who have been needling/rolling for months with no results. On the other hand, about a year and a half after we had started to sell needles/rollers I got a barrage of emails saying they
wanted to give up dermarolling after a year of monthly sessions because there were no significant results but they decided to continue and after a year and a half they finally are getting satisfactory, visible results.