History Colorado won't have legal standing to add licensing or conditions to their use.
They will have the technical ability to control initial distribution. So we can assume he's going to upload highest-resolution originals to their archives. HC may need to delay access to them, or they may only offer downscaled copies for download initially.
They could watermark or alter them in some way before distribution: since being in the public domain really means no restrictions on use or modifications, they can do anything they want upstream. I mean, they probably won't do watermarking, because I don't see it on other images in the "Collection" on the official website. We'll see if EXIF data is processed in any way. Or which formats will be distributed: you could hope for something lossless, right?
But a public domain dedication, properly done, is irrevocable. Fielder may choose to exempt certain works from dedication, for example if a photograph is monetized already, or used by a popular news agency, etc. But it looks like HC will have 5,000 out of 200,000 images, and will shortly release it for public consumption on the same searchable website.
First of all, "free and open source" as in libre, does not necessarily mean "free as in beer". You are perfectly free to pay for FOSS and FOSS makers are free to charge for it. Is that clear?
Second, "FOSS = safe" from a privacy perspective is not nearly the reason you want FOSS. You want a FOSS browser for many, many reasons that are above and beyond privacy assurances.