Yes, Japan's 1889 constitution already guaranteed democracy and free speech. Of course, the 1889 constitution specified that free speech was only permitted "within the limits of law", but in point of fact, all nations call for freedom of speech to remain within the limits of law. There was plenty of censorship before 1947, but plenty of censorship after 1947 as well. And democracy was limited in both constitutions; certainly the 1947 constitution extended suffrage to women, but the mixture of local and proportional representation systems drawn up by the postwar government made it easy for a single liberal conservative party to dominate the entire country.

I would argue that there were three major points of contention with the new constitution, and the "Enlightenment ideal" was the least prominent of them. By far the most important point was and is Article 9, which permanently renounces war. Left-wing Japanese refer to the 1947 constitution as the "Peace Constitution" because it's interpreted as severely curbing the use of international force: Japan is never allowed to initiate hostilities, even when being faced with a life-threatening situation. For the left, this is a legal and cultural solution to the militarism of the 1930s-40s, a harsh medicine to kill the cancer of fascism. To the right, it is a suicidal attack on Japanese sovereignty and the rights of nations generally.

The second controversial point is gender equality. The US doesn't have gender equality in our constitution, but Japan does. For the left, this makes the 1947 constitution a potent legal weapon to be used against patriarchy. For the right, it proves that the constitution is a foreign imposition intended to render Japan effeminate and weak.

Finally there is the question of Enlightenment. Did Japan fail to live up to Enlightenment ideals during WW2? Which ones? The social scientist Maruyama Masao proposed that Europe had shown the world the correct implementation of the Enlightenment and that Japan had strayed from the path and should imitate Europe more closely. Of course, many people objected to this argument, but the general sense of Japan having gone astray did resonate in the 1950s. There was no specific Constitutional mandate connected to this Enlightenment discourse, but and many people looked to progressive mass movements as a way to protect democratic ideals from the small clique of conservative politicians who ran the country. The conservative side, preferring the Enlightened 1889 constitution, did not oppose the Enlightenment generally but rather Maruyama's Europhile interpretation. Only a few extremists, i.e. people to the right of Mishima Yukio, questioned the 1889 constitution alongside the 1947 constitution.

edit to add: Another major change in the 1947 constitution rendered the Emperor a powerless "symbol" rather than the seat of power, but in my understanding, conservatives do not dislike this so much; the power wielded by Hirohito in the 1940s also gave him a dangerous amount of responsibility for Japan's war crimes.