I researched on PowerApps Office365Outlook.SendEmail and Office365Outlook.SendEmailV2 limitations but I haven't found anything related to PowerApps Send Email limitations .
but I can find the limitations for power automate send email that "Send an email notification (V3) action it can only send 100 emails in any 24 hour period. So it will take many days to send 1000 emails. If you use the Send an email (V2) action or the Send an Email from a Shared Mailbox it can send 900 emails every 60 seconds."
but for PowerApps Office365Outlook.SendEmail and SMTP, I haven't found anything related to limitations.
I think that the info you found is a little bit outdated.
Based on specifications,Office365Outlook connector (that include Send an email V2 and Send an Email actions)for PowerAutomate and PowerApps has a throttling limit of 300 API calls in 60 seconds. On the other way, also based on specifications, the Exchange online server has a recipients rate limit of 10.000 per day and a message rate limit of 30 messages per minute.
So, you can send on a rate of maximum 30 emails per minute and you cannot exceed a 10.000 emails per day. Of course, reaching this amount of messages there is a great risk that your mailbox to be on surveillance for spam messages or even worst to be classified as source for this type of activity.