It is important to understand that dual citizenship is something that needs to be evaluated separately under two sets of laws. Malaysian law governs whether you are a Malaysian citizen. The law of the other country governs whether you are a citizen of that country. It is common for the laws of the two countries to be inconsistent. You are a "dual citizen" whenever two countries considers you to be one of their citizens under their citizenship rules.
Even if Malaysia thinks you are not a citizen of the other country because your citizenship in that country was not "acquired by registration, naturalization or other voluntary and formal act (other than marriage) the citizenship of any country outside the Federation", this does not mean that the other country needs a registration, naturalization or other voluntary and formal act other than marriage to become a citizen of that country.
For example, in France, for a very long time (I don't know if it is still true), marriage to a French citizen automatically made you a French citizen by operation of law, but that would not be sufficient grounds for Malaysia to revoke your citizenship.
Similarly, in the United States, you are a citizen by virtue of being born on its soil, even if both of your parents are Malaysian (so long as they are not diplomats or counsels or spouses of the same) and you leave the country, never to return, the next day. Again, having a U.S. birth certificate would not be a basis for Malaysia to revoke your U.S. citizenship, even though the U.S. would consider you to be a citizen based upon your birth certificate alone.
Without knowing both of the countries whose citizenship laws are implicated, it is hard to evaluate.
Your assumption that Malaysia does not allow dual citizenship is not true in all cases, even though it may be true in many cases, and even though it may be hard to continue to be a Malaysian citizen by not committing a voluntary act that forfeits Malaysian citizenship.
Indeed, Section 23(1) acknowledges the possibility of a dual citizenship on its face when it states that:
Any citizen of or over the age of twenty-one years and of sound mind
who is also . . . a citizen of another country . . . .
Also, even if there are grounds to terminate someone's Malaysian citizenship, a Malaysian citizen continues to be a Malaysian citizen until (1) a declaration registered by the Federal Government, or (2) the Federal Government enters an order depriving a person of Malaysian citizenship, that person is still a citizen of Malaysia. This requirement probably couldn't be overridden by a supplementary act or regulation.
For example, if you join ISIS at age 22, and sign a declaration terminating your Malaysian citizenship, but the truck delivering that in the mail to the Malaysian federal government gets blown up by hostile military forces before reaching the postal depot, and your declaration, as a result, never ends up being registered with the federal government, you are still a Malaysian citizen in the eyes of the Malaysian government. (There is a U.S. citizenship case involving very similar facts but a very differently worded statute.)
As @user6726 accurately explains in a comment:
Typically, popular understanding of what the law is, is over 50%