OK Paul, here's a weird one. Just downloaded Teensyduino 1.50. I'm running Mac OS Catalina on a Mac Pro. Been working fine with Arduino 1.8.10 & Teensyduino 1.48. First thing I noticed was the Teensyduino Icon isn't the Teensy board icon, its the icon used for Arduino (circle with an infinity symbol) . Next, if I double-click Teensyduino, it launch an Arduino app that's not the one I have installed. Its fully configured for Teensy, and compiles, launched the Teensy Load app, and downloads. Buts its not the Arduino I have installed. Third: its not a .DMG as you normally post. It's a .app file.
Here's a snippet from ps -ef:
1404 1 /private/var/folders/.../d/Teensyduino.app/Contents/MacOS/Arduino
1407 1404 /private/var/folders/cd/28cyp8nj2v92smmbnd6jy4zw0000gp/T/AppTranslocation/3448F712-A8D1-4D31-B96A-4797BF6799B1/d/Teensyduino.app/Contents/Java/hardware/teensy/../tools/teensy_ports -J2

Process 1404 shows the Arduino app running in a Tensyduino.app. directory buried in a temp directory (/private/var/folders/... where temp stuff is normal stashed on a Mac).

One more clue is looking into the Teensyduino package I downloaded, I see a stub for Arduino which I don't see in the Teensyduino 1.48.
Name:  Teensyduino 1.48.png
Views: 66
Size:  15.3 KB

Name:  Teensyduino 1.50.png
Views: 62
Size:  22.7 KB

Based on the file sizes, my guess is you copied a configured Arduino 1.8.11 with Teensyduino already installed to your web download page