A 70% chance of El Nino means that in 10 similar situations, about 7 would end up in El Nino. It is about likelihood, not intensity.
Forecasts and bulletins
Seasonal forecasts are given as probabilities because the climate has a lot of chaos in it.
70% El Nino does not mean "a 70% strong El Nino". It means El Nino is likely, but a 30% chance of something else remains.
33% for each of three categories would mean "we have no idea" (all equal).
Probabilities for impacts (for example "below-normal rain in Thailand") are always less certain than probabilities for El Nino itself, because many other things affect local weather.