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The ESP32 has 448KB of ROM that embeds most of newlibc as well as wireless drivers and part of the bluetooth stack.

From past experience I can say that tinybasic can go down to about 3-4KB in size.

So it was probably "free" in that they had already decided on a rom size to embed and they had some space left.

Yes, but no.

The big cost saver is doing one design, mass-mass-mass producing it, and then limiting its functionality per market segment.

The boot roms in ESP chips are not particularly small anyway, aside from bootup and flashing functionality, they also embed radio functions for WIFI, Bluetooth, Zigbee/Thread, printf implementations, MD5 functions and more.