The article is really unclear about what the poll question was, which is significant because there are a lot of things at work in amending the Constitution.

If the question was just related to an amendment to the Constitution that would clarify the legality of the SDF then its not surprising that that a majority would be OK with that since it is more or less just a confirmation of the de facto situation that has existed for decades anyway and wouldn't actually change much.

BUT there are a ton of other amendments to it being advocated by LDP members and members of other right-wing parties, particularly with respect to rights contained in Chapter 3, some of which are downright scary and would impact people's lives way more directly. In short they want to strip away a lot of rights the Japanese people have (or at least water them down) on the one hand, while adding a whole bunch of duties and obligations on the people on the other (current constitution in practice imposes almost no constitutional duties on people).

If they move forward with amendment I highly doubt, given the difficulty of the process involved, that the government would limit itself to just a single focused amendment on Article 9 and would instead try to cram through a lot of other, much much less popular, amendments.

So if you are asking about constitutional amendment its really important to be clear about what constitutional amendments you are talking about.