If you buy a train ticket from Eindhoven Centraal to Schiphol Airport, the bus is not included. What is the indicated departure station? Eindhoven Centraal or Eindhoven Airport?

It is cheaper to use an OV-chipcard or OVpay than an e-ticket Eindhoven Airport - Schiphol

An e-ticket Eindhoven Airport - Schiphol costs €28,27.  Train Eindhoven Centraal - Schiphol costs €23,30 full fare, so for the bus you pay €4,97. The ride with bus 401 from Eindhoven Airport to Eindhoven centraal costs €2,74. And if you start at another bus stop it will costs even less. 

There should be no problem using the Airport ticket from or to another line 401 bus stop in Eindhoven but as said, using a credit/debit card for the bus is cheaper.

Unless you arrive at Eindhoven Airport and plan to visit someone in Eindhoven or Veldhoven and continue to Eindhoven Centraal at a later time.