The sum of capacitors in series is less than the one with the smallest capacitance.
The 100pF is a coupling capacitor that couples the capacitance of the diode to the inductor and collector. If the diode's capacitance is 10pF then the total capacitance of the series capacitance is 4.76pF plus: stray wiring capacitance, transistor capacitance and antenna capacitance.
The frequency will change if the +5V changes, if the temperature changes and if something changes the capacitance of the antenna by moving toward or away from it.
The antenna has an impedance of about 75 ohms to ground. When simulating it you need an output coupling capacitor to feed the 75 ohm resistor connected to ground. In my simulations the capacitor-coupled antenna impedance to ground prevents oscillation.