To be fair, though, it is other of those things that Bitcoiners have generally struggled to agree on over the years, heh.
People confuse inflation with the rate of change of inflation. Bitcoin is currently inflationary because the supply is increasing. The rate of change of inflation, however, is decreasing with every halving. Only once the number of coins being irretrievably lost outnumber the number of coins being mined will bitcoin become deflationary. This doesn't necessarily mean we have to wait til 2140. After the 11th halving circa 2052, we will only be generating fewer than 1,300 new BTC each year. All it will take is one major loss a year like the alleged loss of 1,550 BTC last week to push bitcoin in to deflation.
Unless, of course, you consider maximum supply instead of circulating supply. In that case, since you know there will only ever be 20,999,999.9769 BTC, and you consider that any coins lost are subtracting from that total, then you could argue bitcoin is, and always has been, deflationary.