You should be able to resize the model to the real world dimensions. First go into Model Info>Units and set them to Millimeters. Then in the model space, get the Tape Measure tool. Measure the distance between two points making sure you click on both ends. Type the actual dimension and press Enter. SketchUp will ask you if you want to resize the model. Choose the affirmative answer. If you used components in the model, right click on each one and choose Scale Definition to correct the size.
As for why it worked to 3D print your model, the exported .stl file has no units. 35 is just 35. When the .stl is imported into the slicer software the slicer is set up to millimeters and it all plays nicely. As an example, here's a thing I modeled in SketchUp using meters as the units.
Screenshot - 1_9_2022 , 1_44_30 PM.png
And the first rough 3D print. They snap right on to the 18mm dia. tube.
https://live.staticflickr.com/65535/51107454646_b7205eb035_4k.jpg