Here is a quote from Wikipedia (the second version of the Consolations were completed in 1850):
In 1883, years after composing the Consolation, Liszt received a Grand piano from the Steinway Company with a design that included a sostenuto pedal. Liszt began transcribing this Consolation [no. 3] for the new sostenuto pedal and in a letter to Steinway he wrote:
"In relation to the use of your welcome tone-sustaining pedal I inclose two examples: Danse des Sylphes, by Berlioz, and No. 3 of my Consolations. I have today noted down only the introductory bars of both pieces, with this proviso, that, if you desire it, I shall gladly complete the whole transcription, with exact adaptation of your tone-sustaining pedal."
Liszt recommended sparing usage of the sostenuto pedal in the interpretation of this Consolation and opined on the positive effect it would have on the more tranquil passages.
Note that the five-bar note does not mean that Liszt wanted the sustain pedal to be used, because apparently he only got a piano with such a pedal well after composing the piece (the five-bar Db is already in the first edition from 1850 which you can get here).
What does it mean then, that I don't know. After testing it a little bit I would say at least without a sostenuto pedal I would repeat the Db at bar 6 because at that point it starts to sound bad(*). I would still do it so that it does not sound like a repeat but more like just "reinforcing" the sound so it won't die out. You can achieve this effect by playing it much softer than the first Db. So, my interpretation of the ties would actually be more like slurs; there can be many Db-notes but they all belong to one unit.
Anyway, I don't think you should be too worried about it. At the time of Liszt it was common to change the pieces you play so that they fit your taste, technique, mood and whatever. In fact, from the quote you see that Liszt was immediately writing a new version when he got a new piano. Even today a good pianist changes things a bit (especially pedalling, dynamics and tempos) at least to suit the piano and acoustics. If you think re-taking the Db sounds better, you do that. I think it's more important that you experiment with different ways so that you can be fairly sure that it's the best you can come up with.
(*) This is a more general phenomenon: Put the sustaining pedal down and play a strong chord, say C major, with a good supporting bass. Then keep the pedal down and play all kinds of chords on the right hand except C. So far it does not sound so bad. Now repeat the first chord and suddenly it sounds wrong!