1. As explained to you in the Starhub thread, Starhub offer is better.
2. Personally I think it is a short change from Singtel to offer HB611-SGST with 5Gbps or 10Gbps plan. The following is just to explain from Singtel's point of view.
3. Take note Singtel's ONR is the main router for your home network. It is 10G capable with the 10G LAN port. So technically you can use a 10G wired device to get full 5Gbps from the 5Gbps plan. You can get about 8.2Gbps with Singtel 10Gbps plan (same for other ISPs' 10Gbps plan, 8.2Gbps is the practial limit of the XGS-PON technology used).
So Singtel can argue that they can bundle any wireless router as there is no guarantee that using wireless can achieve the full capability of the Fibre internet plan (same for other ISPs).
BTW, Singtel issued wireless router is usually configured in AP mode. In the case of your 5Gbps offering with two HB611-SGST, there are different ways to achieve full 5Gbps.
Option 1
ONR 10G LAN port --> 10G capable wired device, up to 5Gbps
ONR 1G LAN port 1 --> first HB611-SGST, up to 1Gbps
ONR 1G LAN port 2 --> second HB611-SGST, up to 1Gbps
ONR 1G LAN port 3 -->one wired devices, up to 1Gbps
Option 2
ONR 10G LAN port --> first HB611-SGST, up to 2.5Gbps
ONR 1G LAN port 1--> second HB611-SGST, up to 1Gbps
ONR 1G LAN port 2 --> three wired devices, up to 1Gbps
ONR 1G LAN port 3 --> three wired devices, up to 1Gbps
Note: ZTE F8648P ONR will have one more 1G LAN port.
4. Actually it can be worse if you get the Singtel 5Gbps special recontract offer at S$29.90 per month as it comes with free Singtel AX5400 Wireless Router or AP (mesh node) which is capable of only 1Gbps.
https://www.singtel.com/personal/products-services/broadband/eform/recon-dm/fibre-5g
Recontracting to: Singtel Fibre Broadband Plan (24-month contract applies)
5Gbps (WiFi 6) Enhanced Plan at $29.90/mth (U.P. $95.90/mth)
Comes with 1 unit of Singtel AX5400 WiFi 6 mesh device
Singtel 3Gbps new user special offer at S$29.90 per month also comes with Singtel AX5400 device.
https://www.singtel.com/personal/products-services/broadband/eform/3gbps-enhanced-plan
3Gbps Enhanced (WiFi 6) Plan at $29.90/mth (U.P. $84.90/mth)
Comes with 1 unit of Singtel WiFi 6 mesh device
FYI, my recontract for Singtel 5Gbps last August was like this at S$49.90 per month, with ONR plus the free Sintgel AX5400 wireless router.
4. More info: WiFi and Network Configurations for 3Gbps/5Gbps/6Gbps/10Gbps plan
https://forums.hardwarezone.com.sg/...ns-for-3gbps-5gbps-6gbps-10gbps-plan.7073159/
https://www.singtel.com/content/dam/singtel/eshop/Fibre/agreements/5gbps-faq.pdf
5Gbps plans with different setup