These sources are good enough. The reason behind these sources showing less range of BLE than Zigbee is because of the transmission power. the higher the transmission power the greater will be the range. There are a lot of devices which work on 2.4ghz in which your wifi routers are very common and have a big range as well. this causes noise and decreases the range of your device. Now usually wifi routers use channel 1-13. Zigbee uses channel 11-26 and ble uses channel 0-39(Now the problems lies here as well). BLE uses channel 0-36 for data transmission and 37 to 39 to advertise, and not a lot of devices give you all of these channels. HM-10 the easiest available BLE device, has 16 channels which overlaps whith zigbee channels which you can read about in TI cc253x/2540 datasheet,here on page 215. So, if you are using wifi, zigbee and ble in let's say a closed space. Then BLE will have a very short range because of noise and if you try to increase the range by increasing the transmitting power, then it will not remain BLE per se, because if you increase transmission power then you are increasing the power consumption as well, and most ble chips are not meant for that. If you need a big range stick to zigbee or wifi for the time being that's what i will suggest.
First let's clear some things. Did i say transmission power is the only factor? No. Is it a major factor? Yes. Next, sensitivity plays a vital role to receive signal but if you are not able to send signal let's say for more than 5 meters then you cannot expect a higher sensitivity antenna to catch your signals 100 meters away. And if there's a way to do it, please let me know considering it fits inside the BLE chip. To make it more clear read friis transmission equation, i think you will get your answer there.
I hope this answer was helpful to you