No, Tensa Zangetsu's hand guard is not a swastika. It is a mirror image of a swastika. The legs of the symbol open counter-clockwise rather than clockwise.

Interestingly enough, on Japanese maps, reverse swastikas are used to mark locations of Bhuddist temples.

Now for a bit of swastika education:

The swastika is based off of a sanskrit symbol. It was a positive symbol, meaing "good luck, or "wellness."

The swastika is a widely used symbol in many eastern hemisphere countries.
Since Hitler's misuse of it in modern history, in the western hemisphere it's become mainly associated with Nazism.