Because Xuanzong, for most places, gives 'numbers' of buddhist monks and brahmin pujaris. It'd stand to reason that a region with more brahmin pujaris would likely mean the region is hindu majority and vice versa. He also does the same for number of buddhist viharas and 'fire temples'.
Most of the places the chinese record are hard to correlate, because they change it to their chinese pronounciations and then write it in approximations of their symbolic script. For eg, i know Gandhara was written as 'qiantuwowei' and earlier faxian recorded the origin of gupta empire from a place called Milikiasikiapono, which could either be 'mrighashikhavana' or ' mrigasthapana', which would place their start either in UP or Bengal.
In the case of Xuanzong, since he chronicles his journey in chronological fashion, it has been established with fair bit of confidence what each of the places mentioned are.
Also, it'd probably make sense for Xuanzong to think Buddhism had declined in Magadh, given that Pataliputra was destroyed and Magadh came under the rule of the staunch shaivite bengali king Shashanka, after he killed off the Maukhari kings, who were also hindu. Its possible that he was talking about the decline of buddhism in magadh in relation to the destruction of pataliputra and various other centres of buddhism there.
Yes, but that doesnt change my opinion that Sindh was probably Hindu majority - 5th column is most of the time a significant minority population and that makes the probability such imo.
From the period of Chandragupta Vikramaditya to most likely the end of Skandagupta's period, as Western Kshatrapas were known to've ruled from Sindh to Gujrat.
Look up Sassanian Hindh, which was one of the provinces of Sassanian empire.
The confusion comes from what actually was sindh in those times- to the Sassanians, the right bank of Indus was Sindh, while to Indians, the left bank of Indus was Sindh.
This is why the majority of Sassanian coins are found in Larkana-Jacobabad-Sibi region and the Arabs, after defeat by the Gurjar-Pratiharas ( Nagabhatta IIRC) concluded that they re-established posts in Sindh on the right bank of Indus, while the Gurjar inscriptions make it very clear that they drove off the Tajikas & Mlechchas to the other side of Sindh.
The indus being the boundary of the 'scythian kingdom' is mentioned in the Periplus of the erythrian sea ( arabian sea) in Roman sources as well as Shaka kingdom being on the other side of Indus is mentioned in Sassanian records.
Couple that with the rise of the first independent dynasty of sindh- the Rai dynasty- around 470s CE, matches well with the decline of the Gupta empire, where after the death of Skandagupta, the Gupta empire lost its western portions, all the way from Hunza & Gandhara to basically what is India-Pakistan border at Punjab and territories west of the Aravalli range.
Here is a reference to Sassanian hindh ( though the map provided is wrong) :
en.wikipedia.org