As a fellow long-time Tallinn transport user, I can say that this statement can be both true and false. It's true in the strictest sense that yes buses sometimes don't come exactly at the scheduled minute. Also very rarely, but it does happen, a bus doesn't come at all and you need the next one, which is usually 5-10min of waiting.
However when we compare it to other places, I think Tallinn is actually very much among the leaders in reliability. Take something like Italy. It's a miracle if anything arrives as scheduled. Buses are +/- 3min wrong as a standard and often much more. Even the Rome metro doesn't run on schedule! That one is the most bizzare to me, the metro train could just sit in a station and not worry about traffic. However they don't and just speed along, going ahead of schedule.
Then there's the cross-country or even international buses that go through Estonia. Very much on time, sometimes +/- 5-10min of their schedule. Now compare this to something like the US greyhound buses in the northeast. Want to go from New Haven to Boston? Better be prepared for the scenario where your bus is delayed 2 hours because of traffic near NYC.