is it? I always thought that it was just the corrected spelling (a lot of composite words have been merged together in a spelling reform in 1990), and that the English word was actually borrowed from French.

Ha, apparently I’m the uneducated one. I’d assumed it was an anglicisme that happened to work nicely, but it came to English from Middle French. However, the modern tech-related usage certainly first showed up in English, and then was upstreamed to French I assume? That’s kind of amusing… I’ll leave this as a testament to my hubris as a non-native French speaker.

It (the word plateform) never died out in French.

Wiktionary's "etymology" section is great for 98% of words: https://en.wiktionary.org/wiki/platform#Etymology

In general, Modern English has many words from French or other Romance (Latin-origin) languages due to French being an important vector through which Latin-origin words entered English, evolving it in competition with Germanic-origin words.