Taking credit for the last 2MB is a dick move for sure though.
Think what would have happened if he hadn't added the alloc. The team would have been in the same situation, would have busted their guts, got it down under budget and slapped themselves on the back. They might have got it down earlier too: who knows how many person hours got wasted on the last 384K of savings that were never needed.
Think what could have happened with the alloc: the team is hovering around 3-4MB above budget and running out of ideas, with major efforts shedding only a few tens of KB each. They agree they might manage to cut 1-2Mb with a herculean effort, but 4? No way. So with the deadlines approaching decisions are made to cut some feature or content to avoid failing TRCs or missing contracted delivery dates.
If you have to get it down under target or cut something else, then I can't think of a single situation in which having an artificially low goal will help you.
At the very least, the fact that the team thought this guy somehow helped them ship rather than adding to their problems and lauding it over the actual engineers who'd bust their ass to make the savings, is a huge jerk move.
Now, whether or not you actively hide a budget padding is another thing. That is up to the seniors of the project.
This itself implies that the guy WAS one of the "actual engineers" busting their asses.
As a project manager you are also supposed to pad your project schedule a bit without showing your client or engineers the padding.