Perhaps it is the language used, "...AN aerodrome forecast..." implies there was one in the first place but it is unavailable. 'An' is an indefinite article in English and refers to an object in a non-specific way. There has to be an object in the first place, however.
If there was never an aerodrome forecast, the language might be, "when there is NO aerodrome forecast available...". If this were the language i would agree that an alternate would always be required whenever there is no TAF.
This seems to mean that when an aerodrome forecast is unavailable, they mean there sometimes is one but it is unavailable for some reason and an alternate is required.
Any other reading requires a VFR flight to carry an alternate when flying to an aerodrome without a TAF, as noted above.
A careful reading of ENR 58.1.3 reveals that the alternate does not need to have an aerodrome forecast, merely a firm forecast. We all know that the minimum requirement for a place with no IAL is an area forecast. This means the weather at the alternate could be sourced from an ARFOR and the place could be used provided the conditions are above the appropriate alternate minima and the place does not require an alternate for an unrelated reason.
What was the RFDS answer?