@Icy13 - this plugin never really got beyond being 'experimental', which is why it's not in the Plugin Index, and given the author hasn't posted since April last year I'd say he/she has abandoned it.
For TTS I use the Balabolka program, which I can access via calibre's built Open With feature. It also has the advantage of being usable directly.
And, whenever you read non ePub formats (e.g. PDF) in the calibre viewer it converts the file to ePub which can generate a large quantities of redundant markup. I would guess this PI parses the ePub skipping the markup.
BR
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