Some people assume that since bitcoins are still mined, and the supply is increasing rather than increasing it's right to say that Bitcoin is inflationary, but we have to consider the fact that when we say it's deflationary, we actually mean the number of bitcoins mined keep decreasing. We aren't referring to increase in supply of bitcoins.
This is correct, but the main reason bitcoin is deflationary are two things and they are bitcoin having a supply of just 21 million BTC which is limited and the second reason is because bitcoin has value/demand. People see bitcoin to be a store of value and they are demanding for it which helps in its adoption.