Just tried it but it's not a direct import - it's "dump your data from Instagram as JSON[1] and we'll import that JSON". i.e. a one-off backup restore.
(If ActivityPub allowed backdated posts, I'd do the sync myself from my instaloader downloads but alas, most servers do not.)
[1] "It may take up to 14 days to collect this information and send it to you."
If you leave that tab at all, it breaks and restarts which cancels the import process.
Once it's uploaded the file, Pixelfed wants you to tap each image you want to import in a 3-wide scrolling grid and there's no "select all" I can see. Plus it is now DOG SLOW.
I have 9052 Instagram posts. Bugger that, frankly, because I'm not spending upwards of 3 hours clicking on a web page to import photos.
In summary - nice idea badly implemented.
[edit: Tried it in Chrome. Upload is much smoother and more reliable but still no "select all" and it still chugs like a 1970s PC - a good 30s between clicking and the image being selected.]