While upgrading phan for this extension (and here comes its usefulness!), I discovered that MappingConfigBuilder.php is incredibly old.
To give an idea, it still uses $this->buildLongField() [note, the following applies to similar methods too]. However:

1. This method, inherited from CirrusSearch\Maintenance\MappingConfigBuilder, was removed in rECIR8996d2f5e88ef6295624a0427a2cc7009f0c66e9, in 2017, in favour of a new component;
2. It requires a parameter to be passed in since rECIRd08773fecfa83e4146ece86b3651b87afb52b982, in 2016. And it stopped returning a plain array.

This means that the script isn't working since 2016, and thus I guess it should be either updated, or just deleted. Either way, I'm not touching it.