How a boot loop prevents access to TWRP?
A boot loop can be caused by several reasons.
• Incompatible Boot and Recovery Image's.
• System Modifications
• File Permission Errors
• File Owner Errors
• Incorrect Formatting Of Files
• Knox 
And the last one i can think of is incorrect settings ( build.prop edits etc.. )   

Yours seems to be the first one, or the last, Incompatible Recovery and Knox

Obviously you have had a working TWRP on that device before, so maybe you still have it from flashing the device years ago. 
Custom firmware like Lineage OS can also require it's own recovery image, that is dependent on the corresponding Kernel version.  

If your Lineage OS installation was for an UPGRADED Android version like 4.4 to 5.1, then a new kernel for 5.1 will have overwritten your 4.4 kernel, and now that possible working TWRP is useless... This became a huge problem with people trying to downgrade device's, as it requires a factory level flash. Odin may have the ability the downgrade the devices boot image ( kernel ). and TWRP will work again, but Knox will stay triggered.