Dark ages meant that there is a sharp decline in the quality of life especially in the knowledge and scholarly departments.
If the Sultanates are really bad as you have described there should be no Hindu left in the India sub-continent but the last time I checked they are still the majority. The Sultanates can easily pull a Spanish Inquisition style by killing or forced conversion. They can drive away the Hindu away like what the Spanish Christians did during inquisition to the Muslim and Jews by not spare anyone of them and eliminated all the mosques and sinagogues from Spain[2].