Yes exactly. The first text-based email clients I used that quoted the original message in a reply[1] defaulted to bottom posting. They'd include the message with the ">" prefix on each line, and your cursor would be positioned at the bottom of that. So at that time most people did inline or bottom replies.
Microsoft Email/Exchange/Outlook as best I can remember have always defaulted to quoting the message and leaving the cursor at the top. And they didn't prefix the quoted text in any way, so to do inline replies you'd have to make it clear which text was quoted and which text was new. People would use colors or different fonts when "rich" text became supported but it was more work and most people just took the easy path.
[1] Very early on, MUAs did not (at least by default) quote the original message. This was because most people were using teletypes or slow (300 baud) terminals and possibly a line-oriented editor such as 'ed' so quoting the original message could add significant extra time needed to compose a reply. Also, with a mail client that does proper threading, you can see which message is being replied to and you don't really need to include that again in the reply.