I don't believe anyone was promised anything but there are lots of reasons why the staff would go with the 5th-year senior rather than the younger guy that they inherited, even if the competition was close as the staff has claimed, without any promises being given out. They might not be good or wholly logical reasons from our perspective but who ever accused people of always being logical?
For one, I think when you have a new staff, there is a always a strong desire on their part to go with their own guy, to win or lose with the guy they brought in over what they inherited. It doesn't always work out that way (see Nassib) but there often does seem to be a house-cleaning stage in a coaching transition and the qbs on the roster are usually the ones who suffer most.
Second, when you bring in a fifth year guy, you don't bring him in with the intention of sitting him. They probably feel that they have an obligation to play him and may give him a bit more rope to fail than the other guys. BUT that's not necessarily out of any kind of obligation that they feel towards him. Rather, it's due more to the faith they place (and must place) in their own evaluations. They recruited him because they felt that on film he was a clear step up from what they already had on campus. The problem there was they didn't have much film to go on to make that evaluation and the only film they had of the qbs on roster were from practices. So, yes, you have to have faith in your evaluations but they should recognize that the situation here was different than with a HS qb with tons of game film from which you can project their future development or an experienced lower-level college qb who's looking to take a step up the ladder and transfer to a bigger program.
Third, I think playing the fifth year guy can from their perspective be helpful in recruiting. Now they can tell top QB recruits that the position is wide open next year. With Hunt, anyone coming in may be forced to wait behind a potential 3-year starter. The problem there is that if they keep losing with the fifth-year guy, then recruiting is generally not going to benefit from it.
So there may be other reasons for going with Allen that have nothing to do with a promise. They each have their own logic but some of it may not be well thought out. But who ever accused coaches of thinking things out clearly?