True, although I'd be remiss if I didn't note that, despite the stereotypes, politicians do keep (or at least try to keep) most of their promises. FiveThirtyEight put out an article a while back which showed that, when a politician makes a campaign promise - even when it isn't a close fit with their ideological background - they actually do try to fulfill it. So, at least when taking that into consideration, it's good news that Biden's promises are to the left of Hillary's & Obama's.
I definitely agree that platforms matter. The reason Bernie is keeping his name on the ballot (and I really hope that all his supporters keep voting for him in the remaining primaries, at least provided that it's safe to do so) is to have as much influence as possible on the Democratic platform. And fighting for that will, in a general sense, influence the future course of the party.
With regard to Biden's specific campaign promises, I don't doubt that he will
try to pass healthcare reform, environmental legislation, and other aspects of his platform. It would be crazy for any Democrat not to. The main thing is that I don't trust him to fight for these things as hard as Bernie (or Warren, or frankly even someone like Kamala), because I believe he's indicated that he will prioritize other goals (including, and that's the most damning thing about Biden, f**king comity with Republicans). Every President only has so much political capital and so much energy (and not every President has the same amount of energy to begin with), so how you use it is more crucial than anything else. If we were in a parliamentary system where policymaking authority clearly rests in whoever wins the election, then I'd be much more confident that good things would come out of a Biden presidency. But in the US, what policies a President promises bears a much looser relation to what actually gets done. All that is a digression from the question of ideology, though.