xcodebuild archive failure: picking the wrong SDK?

I'm using fastlane to release an app. The simplified version of xcodebuild command to build the app is this:

xcodebuild -workspace App.xcworkspace -scheme App 
           -configuration Release -sdk iphoneos13.6 
           -destination 'generic/platform=iOS' clean archive

This fails giving (adding a sample):

/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:807:2: error: Unsupported architecture
#error Unsupported architecture
 ^
In file included from /Users/sudeepkumar/Zendrive/mobile-apps/ios/copilot/Pods/glog/src/symbolize.cc:55:
In file included from /Users/sudeepkumar/Zendrive/mobile-apps/ios/copilot/Pods/glog/src/utilities.h:73:
In file included from /Users/sudeepkumar/Zendrive/mobile-apps/ios/copilot/Pods/glog/src/base/mutex.h:141:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/pthread.h:55:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/_types.h:27:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/_types.h:33:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/machine/_types.h:34:2: error: architecture not supported
#error architecture not supported
 ^

I see that it's picking the MacOSX.sdk, I would expect it to pick iPhoneOS sdk present in Xcode directory. Is this the reason for failure? Or something else?

xcrun output:

» xcrun --show-sdk-platform-path
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform

» xcrun --sdk iphoneos --show-sdk-platform-path
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform

from xcodebuild archive failure: picking the wrong SDK?