No, resigned from PAP one year after the he retired in 2008... You must resign from all political parties to be President.
Ah, 2008 was the year he where he came up with the petition for President idea.
Reference: http://sammyboy.com/showthread.php?96177-TKL-From-Petition-to-Fundraising-with-Intimidation
So, cadreship has served its purpose for ensuring that he remains as the top person in NTUC and upon his golden handshake there, there is no longer any use for being a cadre anymore, and there he resigns so as to get higher ranking job, ie. President... pardon me for being so direct.
Another question, since he is not a PAP-cadre reject, what is the explanation for his post 2008 resentment against the PAP? He was not given the opportunity to be an MP despite his loyalty?
The plot thickens.