Do I need to make a reservation to have lunch in the lounge?

No need to make reservations

I would recommend making reservations but you can also just walk in.

I've been there dozens of times as long as you don't mind sitting in the lounge and it's afternoon like after 2 and before 5 it's not a problem

I would not advise going without a reservation. The online process is ready and straightforward. The day i was there had plenty of vacant tables for lunch, so you may be OK to turn up. But why risk it when reserving a table is so easy?