I'm sure modern PHP code does not prevent scalability, but old PHP code definitely does.
Also, and this is slightly beside the point, but last time I tried Nextcloud there were _usability_ "scalability" issues, such as being unable to upload large files in the web GUI due to classic PHP config issues such as client timeout and upload file size limits.
Failure to account for modern file size limits is a massive code base red flag.
Sure, it's a doddle to fix if you know basic PHP, but it's a sign that the developer doesn't understand the core requirements of the product they are developing.