Yes, I have the same question. The posted circuit is a constant-voltage charging circuit; not a constant-current method required by Nicads.
Nicads are probably the worst battery technology to use to back-up a real time clock. The useful life of the Nicad even under constant-current trickle charge will be less than the shelf life of the Cr2025 it replaces.
A better approach is to diode isolate the Cr2025 so that while the 5V is available, the operating current for the RTC comes from the 5V, and the battery supplies current to the RTC only in the absence of the 5V.
If you insist on using the Nicad, just calculate a resistor that will limit the charging current into the Nicad to less than the 0.05C rate. Under long term charging, the Nicad's terminal voltage will likely climb higher than the stated 3.6V, say 3.8V, so if the capacity (C) of the Nicad is 0.06Ah, the 0.05C rate would be 3mA, so R=(5-3.8)/0.003 = 400Ω