Hi Sanjeetha,
The digital modulation (QAM) you are referring to is a type of “digital” modulation; the digital data is “coded” by a coding scheme to enter the channel (in the example provided in the link, the system even works in electrical domain). For example by QAM modulation in your case. In the case of LED modulation, “modulation” refers to the modulation of electrical signal into optical signal. So they are completely different from one another and different methods of each modulation should be compared with the other method of its own modulation type (optical or digital).
There are two major groups of optical modulations: Direct modulation and external modulation. The one provided in the link is direct optical modulation. This is not directly related to your question, but If you need to know more, please let me know.
Regards