That is true for many countries, but not for all. The US, for instance, insists that its citizens use a US passport to enter the US. South Africa and, in almost all instances, Australia have similar requirements.

But the UK, for example, does not, and merely requires that the documentation presented at a UK port of entry is adequate for the purpose of the trip. So there is nothing to stop, say, a dual Botswanan-British citizen going to the UK on a holiday using his or her Botswanan passport and entering the UK as a Botswanan citizen. Generally, though, the only thing that the UK will put in the non-British passport of a British citizen is a right of abode certificate: they won't knowingly issue a visa to a British citizen.

Other countries that do not in all circumstances require their citizens to use the "home" passport are quite numerous, and include Ireland, France, New Zealand, Canada...