Can I add output capacitance to a dc/dc boost converter to help with current spikes?
Your boost converter is not 600W. 

It says 

Output power: = Input voltage * 10A


in the eBay listing. Therefore, 10A*12V = 120W output power. The laptop you have can use up to 19.5V*9.23A = 175.37W of power. If it does, the boost converter can't keep up and limit its output so laptop will see voltage reduction and switch to internal battery. 

Capacitors can help, but only if average load is lower than whatever boost converter can provide.