As you have clearly pointed that SDP had already given way to NSP in 2015 and 2020. This means that they are no longer interested in these areas.SDP has a history of contesting in Sembawang GRC but they gave way to NSP in GE2015 and GE2020. NSP's performance was abysmal, even worse than SDP's. Now that there's a new SMC in Sembawang, NSP will definitely lose badly in a straight fight with PAP and it would be like a walkover. It's such a wasted opportunity. With SDP contesting, PAP has to wake up and put in some effort.
Last GE, NSP fielded two 70+ year old uncles in Sembawang GRC. NSP's uncle candidates hardly ever walked the ground and it doesn't even look like they are trying to win. NSP has been going downhill since GE2011 and it's better for a stronger party to contest in those areas, even if there's a multi-cornered fight.
It's doesn't matter whether NSP is weak or not. The question is about the integrity of SDP. SDP came back to Sembawang because Bukit Batok merged into a GRC. Chee decided to fight in a SMC. Why don't Chee fight in the GRC where Bukit Batok is merged into? SDP has already left the area since 2015. And now they back paddled because of their own interests, and not the voters.
Furthermore, where do SDP think that NSP should go to if they wanted to fight here? Worst of all, a mutli-corner fight will see both SDP and NSP defeated. Does Chee think highly of himself that the voters will choose him over NSP and PAP?