MTBF means that, in the expected lifetime of all devices combined, the expected time between failures is 2.5 million hours.

So, if you buy 3000, one will have a failure within about a month (assuming expected lifetime is larger, but that is not stretching it)

Also, MTBF says nothing about what happens outside the expected lifetime. If that is 10 years, whether they all break down on the first day of year 11 or somewhere between year 10 and year 100 doesn't affect MTBF.

So, what do they expect the lifetime of these devices to be?