This is how it has always worked. Older iOS devices tend to not support everything newer iOS devices support when they're updated, due to older and weaker hardware. It's nothing new, and to be expected, as weaker hardware has worse performance. This is especially true with the Neural Engine and GPU, considering the NE and GPU in older SoCs are significantly weaker than what is included on the A17 Pro.The problem with this 'commitment' is it does not mean that iPhones will have full facilities, i.e. AI for example, where it suggests that even phones way under 5 years will find they can not utilise functions otherwise available.
Effectively it might support software upgrades, but not necessarily full upgrades across the 5 year range, so by recent comments about AI, it would make that 5 year support somewhat erroneous.