Why signal can't travel between ports of such double balanced mixers?

[Terminator3]

Although it is high frequency design, my question relates only to low frequency part.
Recently i learned that mixers must have high pass filter:

The high-pass filter HP must pass both the high-frequency RF signal and the
oscillator signal, and suppress the intermediate IF frequency, which is assumed to be
much lower than the RF and the LO frequencies

https://www.qsl.net/va3iul/RF Mixers/RF_Mixers.pdf
But in many high frequency mixers i can't presence of this HPF (High Pass Filter) which must block this IF low frequency.

For example, in this transmit mixer we apply low frequency modulation signal (around few kHz range) to ports 1 and 2. This two modulation signals usually have different phase to realize SSB modulation. But i can't see anything that prevents this modulation signals to travel between ports 1 and 2 through the path i shown with red line:


The same in receive mixer:

Why mixer diode output from diodes "1" can't travel through microstrip line up to the port "2" and vice versa? All quarterwave stubs and other microwave stuff does not apply to this low frequency signals.

I can't find explanation why this and many other similar mixers have isolated two IF ports at low-frequency AC. Diodes have some threshold, but also they are constantly "biased" with LO signal. So i can't explain this with the fact that IF signal have too small amplitude to go through diodes.

 

[chuckey]

In the first diagram the output port has a 2.2µH choke shorting out the LF signals, so they don't flow across the circuit.
Frank

 

[Terminator3]

thank you very much. I see second scheme have connection to ground too. I alway thought it is only for biasing diode.