I think the early wakeup is already noted in the initial PR for the pico2 (#15619 (comment)):

here are some issues with lightsleep mode on ARM

I can confirm the same behavior on Pico W (RP2040) with latest Micropython stable release (v1.24.0).

I have slightly modified this example to:

# Lightsleep MicroPython example.
from machine import Pin
import machine
import time
led = Pin('LED', Pin.OUT)

while True:
    for _ in range(5):
        led.toggle()
        time.sleep_ms(200)
    time.sleep(1)
    machine.lightsleep(5000)    

On older Micropython Releases (v1.22.0 & v1.23.0) the above code has an expected delay of 6 seconds between the short pulses.

Using the latest Micropython Release (v1.24.0), the same code has only a 1 second delay between the short pulses - the lightsleep instruction is "ignored".

Sidenote:
When replacing machine.lightsleep with machine.deepsleep, there is also only a 1 second delay after the short pulses. Then the pico will reset. This can be observed as for the longer 1 second delay the LED is always "on".