(Edit: This post was drafted and posted while the earlier replies to the OP took place).
If your source means if Openreach (or any other such as CityFibre) install a fibre link to your house (FTTP) then yes it is true in that the connection speed would be exactly as advertised in both directions. Whereas your current FTTC connection speed depends on the length and quality of the copper (sometimes aluminium) between you and the cabinet, as I expect you know.
The actual throughput speed you get is still subject to a small amount of overheads and the individual provider's capacity at the exchange plus other factors, but most of the time is not an issue. On the 40/10 product I would expect almost full speed on a speed test. Also on the higher 900Mbps products.
An extreme case of course would be if I had an internet server in my house with my website on it. The throughput speed you got if you visited my site would be limited by my upload connection speed and how many others were accessing it at the same time

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Connections: OnePlus 8 Pro on Three 4+ (LTE)/5G and at home Three Mobile, with (Three)ZTE MF286D router giving about 113/20Mbps.
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“I have hardly ever known a mathematician who was capable of reasoning.” (Plato)
Edited by pluralist (Mon 24-Jan-22 14:18:22)