The 555 timer is considered one of the most successful chip designs in electronics. It has a simple, well understood, design. The timing on the 555 can be configured to generate a periodic signal using just a few resistors and capacitors. The 555 is often used as a timer in electronic circuits. The Linux scheduler is usually configured to respond to interrupts generated by a cpu. To replace those cpu interrupts by a simple 555 timer is impressive due to the unconventional nature of the setup.

I don't think it's a flex per se, but more of a nod to where many people start off in their electronics careers. The LM555 has been used for four(?) decades to teach electronics to students.

A cheap $1 8-pin microcontroller (attiny, avr8, pic) could do the job of an LM555 without the LM555's required capacitors/resistors. But that's not as fun.

There's a bit of a meme among electronics hobbyists that for so many projects with an Arduino, somebody in the comments says "I could've done that with a 555”. It's a very common chip with a wide range of uses from arcade sound generation to the timer on a toaster to light/motion activated alarms.

There is something about the 555 that triggers love from people